Update 6.30pm: Russia will refuse to meet Theresa May's midnight deadline unless Britain agrees to send Moscow samples of the nerve agent used to poison Sergei Skripal.
The country's embassy in the UK fired off a salvo of seven tweets in which it said Britain must comply with the Chemical Weapons Convention for a joint investigation and warned the threat of sanctions would "meet with a response".
It comes as President Donald Trump told Mrs May in a phone call the US is "with the UK all the way", according to Downing Street.
Mrs May said the Government had concluded it is "highly likely" Russia was responsible for the attack which left ex-spy Mr Skripal and his daughter Yulia in a critical condition in hospital.
She demanded that Moscow account for how a Russian-produced nerve agent could have been deployed in Salisbury and vowed to set out measures Britain will adopt if no credible response is received by the end of Tuesday.
Home Secretary Amber Rudd earlier said Russia had "started responding" but the embassy appeared to suggest this amounted to little more than informing the Foreign Office of its demands and reiterating it was not involved.
Britain's ambassador to Russia was summoned by Moscow and told by first deputy foreign minister Vladimir Titov that the Kremlin "strongly protested" the accusations, the embassy said.
Update 4.07pm: Salisbury spy probe could take weeks as investigators carry out 'painstaking' inquiry on nerve agent
The Salisbury spy probe could take several weeks as investigators carry out a "painstaking" operation to identify how a nerve agent was used to poison a former double agent on British soil.
Giving the first investigative update on the case since Theresa May publicly pointed the finger at Russia, counter-terror police chief Neil Basu said officers' "prime focus" is to establish the method used to administer the chemical weapon.
A huge police inquiry was launched after former double agent Sergei Skripal, 66, and his daughter Yulia, 33, were found slumped on a bench in Salisbury, Wiltshire, on March 4. They remain in a critical condition.
Detective Sergeant Nick Bailey, who was part of the initial police response, was also taken ill. He is in a serious but stable condition.
Speaking at Scotland Yard, Mr Basu said: "The public are going to continue to see a great deal of police activity in and around the city, including potentially more cordons being erected, but please don't be alarmed.
"It is necessary as part of this major investigation by the counter-terrorism policing network. In truth it may last many weeks."
The Metropolitan Police Assistant Commissioner would not identify any potential suspect, saying: "It's a painstaking operation to identify anyone of interest to this inquiry, and eliminate them or include them, but at this stage we are not declaring a person of interest or a suspect."
Investigators are also focusing on Mr Skripal's red BMW, registration number HD09 WAO, and appealing for any witnesses who saw the pair in the car between 1pm and 1.45pm on March 4 to come forward.
Police have so far collected 380 exhibits and have been scouring hours of CCTV footage from across the city.
Detailing the timeline leading up to the pair being taken ill, Mr Basu disclosed that Yulia had arrived at Heathrow Airport on a flight from Russia the day before.
The senior officer referred to Mr Skripal as a British national and Yulia as a Russian citizen.
He also revealed that a total of 38 people had been seen by medics in relation to the incident, of whom 34 had been assessed and discharged from hospital.
Three people remain in hospital - the Skripals and Mr Bailey. One other unnamed person is being monitored as an outpatient but is not showing signs of exposure to the chemical weapon.
As the clock ticked towards the midnight deadline Theresa May set for Moscow to explain how a military-grade chemical from a group of nerve agents known as Novichok came to be used on British soil:
The episode has left Britain's relations with Moscow, which were already under severe strain, at breaking point.
On Monday, Mrs May said the Government had concluded it is "highly likely" Russia was responsible.
She demanded that Moscow account for how a Russian-produced nerve agent could have been deployed in Salisbury and vowed to set out a "full range" of measures Britain will adopt if no credible response is received by the end of Tuesday.
Update 2.51pm:President Trump has appeared to back the British Government in the row with Moscow over the Salisbury nerve gas attack, saying: "It sounds to me like it would be Russia based on all of the evidence they have."
He was questioned after he sacked Secretary of State Rex Tillerson just hours after Mr Tillerson strongly supported the UK.
The President told reporters outside the White House: "Theresa May is going to be speaking to me today.
"It sounds to me like they believe it was Russia and I would certainly take that finding as fact."
The President, who had initially avoided blaming the Kremlin over the assassination attempt against former double agent Sergei Skripal, had announced on Twitter that Mr Tillerson would be replaced by CIA director Mike Pompeo.
The Washington Post reported that the decision was made on Friday and Mr Trump said he and Mr Tillerson, who had reportedly once called the President a 'moron', had been "talking about this for a long time" and had disagreed on issues like the Iran deal.
But with the Trump camp's links with Moscow facing continued scrutiny, the timing of the announcement prompted questions about the President's support for the UK in the row with Russia.
Update 1.52pm: Britain needs to be "much, much tougher" in its response to Russia over the Salisbury nerve agent attack, a prominent Tory MP has said.
Sir Nicholas Soames, the grandson of Sir Winston Churchill, said any decision not to send "a lot of dignitaries" to the World Cup, due to take place this summer in Russia, was "nowhere near good enough" and was a "pathetic response".
Speaking during a Westminster Hall debate on the Diplomatic Service and resources, Sir Nicholas (Mid Sussex) was asked by the DUP's Jim Shannon (Strangford) if withdrawing from the World Cup and having it in England would be an impressive way of putting pressure on Russia to change.
Sir Nicholas responded: "I don't think that it's nearly serious enough for the kind of steps that I believe this Government will need to take against Russia. I think just to say that you're not going to send a lot of dignitaries to the World Cup is nowhere near good enough, it's a pathetic response.
"We will need to do much, much better and be much, much tougher than that so that they understand across the full spectrum that this is the kind of behaviour up with which we will not put."
He joked that he used to say: "The Treasury do work for the Russians (in) the way that they have undermined successfully our military effort."
Sir Nicholas branded Brexit a "very poor" decision", arguing there was a compelling and unassailable case for Britain to retain and develop its active diplomacy.
Britain, he said, was at a crossroads, adding: "Our global influence is already coming under very considerable pressure and it is essential for the further success, safety and security of this realm that our diplomacy is properly resourced."
He warned the country was not a superpower but a middle-ranking one, arguing it was "inevitable that our influence - already, sadly but quite clearly, on the wane - will inevitably further decrease as the realities of the folly of our exit from the EU become clear".
He added: "We're going to have to redouble all our efforts, we're going to have to call in our chips and we're going to have to work very, very hard to retain our influence and position on the world stage."
Tory former minister Richard Benyon (Newbury) who brought the debate, said: "Brexit brings a new urgency to our deliberations. It's not too late to see a paradigm shift in our strategy, but influence is hard won and is easily lost."
Mr Shannon argued the UK could "thrive" as a nation post Brexit by becoming a global Britain, but needed the diplomatic services and the resources to make that happen.
Shadow foreign and Commonwealth affairs minister Helen Goodman argued there had been a "significant" reduction in the department's budget.
My Question to the Prime Minister in her Statement on Monday about the attempted murder in Salisbury by poisoning. https://t.co/HnC5SBU6CT— Nicholas Soames (@NSoames) March 13, 2018
Foreign and Commonwealth Office minister Mark Field said he had a "significant amount of sympathy" with Sir Nicholas's comments, adding that Brexit was a mistake, but "we have to make it work".
He said: "If we are to maintain and increase our global outreach and our influence we need to make sure that the vision of a global Britain is more than just a mantra.
"I accept that this must ensure that we provide the investment that is required.
"The FCO will continue to work closely with the Treasury to ensure the Diplomatic Service is sufficiently resourced."
Update 11.56am: Russia has asked for access to samples of the nerve agent that poisoned ex-spy Sergei Skripal and his daughter Yulia in Salisbury, Foreign Minister Sergei Lavrov has said.
Mr Lavrov said Russia "is not to blame" for the incident on March 4.
The Russian Foreign Ministry has summoned the British ambassador in Moscow over the case, Russian news agencies have said.
Mr Lavrov warned that Russia will only co-operate with Britain on the investigation if it receives samples of the nerve agent that is believed to have been used to target Mr Skripal and his daughter.
Mr Lavrov said that Moscow's requests to see samples of the nerve agent have been turned down, which he called a violation of the Chemical Weapons Convention, which outlaws the production of chemical weapons.
Prime Minister Theresa May said on Monday that Mr Skripal and his daughter had been poisoned with a military-grade nerve agent of a type developed in the Soviet Union.
She said Russia has until the end of Tuesday to explain how the substance ended up in Britain.
Mr Lavrov said on Tuesday that Moscow is willing to co-operate with the probe, but he suggested London would be "better off" complying with its international obligations "before putting forward ultimatums".
Update 10.42am: Russia's foreign ministry has mocked Theresa May over her conclusion that it was "highly likely" Moscow was responsible for the nerve agent attack on Sergei Skripal and his daughter Yulia.
In a post on Twitter, the ministry's official account used the hashtag #HighlyLikelyRussia - which has been used on social media as the basis of jokes for things to blame Moscow for.
The ministry of foreign affairs' Tweet said "sincere thanks to Mrs May for #HighlyLikelyRussia" along with a video suggesting the country was to blame for the recent snow to fall in the UK.
Sincere thanks to Mrs May for #HighlyLikelyRussiaMarch 12, 2018
It came as the ministry - through its ambassador in London Alexander Yakovenko - faced a midnight deadline to explain how military-grade Russian nerve agent came to be used in the Salisbury incident.
The Kremlin has denied responsibility and Moscow's response has been to appear to taunt the UK.
Kirill Kleymenov, a presenter on Russian state TV, advised "traitors" against moving to Britain, adding: "Something is wrong there. Maybe it's the climate, but in recent years there have been too many strange incidents with grave outcomes there."
And another prominent broadcaster, Dmitry Kiselyov, suggested that the UK may have been behind the poisoning of the former double agent.
As a source, Mr Skripal was of little value, but "as a poisoning victim he is very useful" to harden British attitudes against Russia, he said.
Earlier:The nerve agent attack on ex-spy Sergei Skripal and his daughter Yulia "clearly came from Russia" and "certainly will trigger a response", US Secretary of State Rex Tillerson has said.
Mr Tillerson's comments amount to the strongest US response yet to Theresa May's declaration that it was "highly likely" Russia was behind the horrific poisoning in Salisbury on March 4.
The British Prime Minister told MPs that the highly dangerous substance used in the attack was a military-grade Novichok nerve agent produced by Russia.
She set a deadline of midnight today for Moscow to explain whether it was behind the attack or had lost control of its stockpile of the poison.
Failure to provide a "credible" response would lead her to view the incident as "an unlawful use of force by the Russian State against the United Kingdom", sparking unspecified measures in reprisal.
Home Secretary Amber Rudd will chair a meeting of the Government's Cobra emergencies committee in Whitehall on Tuesday morning to discuss the latest developments.
According to AP, Mr Tillerson told journalists travelling with him in Africa that the Novichok agent was "only in the hands of a very, very limited number of parties".
Although he said it "clearly came from Russia", he added that he did not know whether Vladimir Putin's government had knowledge of the poisoning and said it was "almost beyond comprehension" that a state actor would use such a dangerous substance in a public place.
In a formal statement released after a phone call with Foreign Secretary Boris Johnson, the US Secretary of State said: "We have full confidence in the UK's investigation and its assessment that Russia was likely responsible for the nerve agent attack that took place in Salisbury last week.
"Those responsible - both those who committed the crime and those who ordered it - must face appropriately serious consequences. We stand in solidarity with our Allies in the United Kingdom and will continue to co-ordinate closely our responses."
With the world weighing up the possibility of sanctions against Russia, French president Emmanuel Macron offered his country's solidarity with the UK in a phone call with Mrs May, in which he said that Paris would "co-ordinate closely" with London following Russia's response.
A Downing Street spokesman said: "They discussed the wide pattern of aggressive Russian behaviour and agreed that it would be important to continue to act in concert with allies to address it."
Nato secretary general Jens Stoltenberg said: "The use of any nerve agent is horrendous and completely unacceptable.
"The UK is a highly valued ally, and this incident is of great concern to Nato. Nato is in touch with the UK authorities on this issue."
Mrs May's dramatic statement to the Commons on Monday came after Mr Johnson summoned Russian ambassador Alexander Yakovenko to the Foreign Office to voice Britain's outrage, giving him little more than 24 hours to provide Moscow's response.
The Kremlin has denied any involvement in the nerve agent attack on Mr Skripal, a former Russian intelligence officer who was jailed as a double agent before being sent to the West in a 2010 spy swap.
Following Mrs May's statement, news agency Tass quoted Russian foreign ministry spokeswoman Maria Zakharova as saying: "It is a circus show in the British Parliament.
"The conclusion is obvious, it's another political information campaign, based on a provocation."
And Mr Putin dismissed questions about the Skripals when he was confronted during an election campaign visit, telling the BBC: "Get to the bottom of things there, then we'll discuss this."
Mrs May said: "On Wednesday we will consider in detail the response from the Russian State.
"Should there be no credible response, we will conclude that this action amounts to an unlawful use of force by the Russian State against the United Kingdom."
That would result in Mrs May setting out "the full range of measures that we will take in response".
The National Security Council is expected to meet on Wednesday to discuss the Russian response, if any, ahead of a statement by the PM.
Mrs May said: "This attempted murder using a weapons-grade nerve agent in a British town was not just a crime against the Skripals.
"It was an indiscriminate and reckless act against the United Kingdom, putting the lives of innocent civilians at risk.
"And we will not tolerate such a brazen attempt to murder innocent civilians on our soil."
In the US administration's first public statement on the issue, White House press secretary Sarah Huckabee Sanders said the attack was "reckless, indiscriminate and irresponsible".
She said: "The use of a highly lethal nerve agent against UK citizens on UK soil is an outrage.
"We offer the fullest condemnation and we extend our sympathy to the victims and their families and our support to the UK Government.
"We stand by our closest ally and the special relationship that we have."