Foreign Secretary Boris Johnson has welcomed international support over the Salisbury nerve agent poisoning as the deadline for Russia to provide a "convincing explanation" creeps closer.
France, Germany and US Secretary of State Rex Tillerson all gave their backing to the UK after the Government confirmed it was "highly likely" Russia was behind the use of deadly Novichok on UK soil.
However, President Donald Trump's White House press secretary Sarah Huckabee Sanders stopped short of pointing the finger at Russia.
Speaking on Tuesday Mr Johnson said: "What we're doing today is giving Russia until midnight tonight to explain how it came to be that Novichok was used on the streets of Wiltshire.
"If they can come up with a convincing explanation then obviously we will want to see full disclosure of that to the Organisation for the Prohibition of Chemical Weapons in the Hague.
"If not, then clearly we will want to be announcing the UK response and that will come tomorrow.
"In the meantime what we've been doing is talking to friends and partners, explaining what we see as the high likelihood of Russian state agency.
"And I've been very encouraged so far by the strength of the support that we are getting.
"I've been encouraged by the willingness of our friends to show support and solidarity."
Mr Johnson added: "Russia is a great country.
"It is a great pity therefore that the Russian regime seems to be moving in this dangerous and disruptive direction."
A huge police inquiry was launched after former Russian double agent Sergei Skripal, 66, and his daughter Yulia, 33, were found slumped on a bench in Salisbury, Wiltshire, on March 4.
The pair are in a critical but stable condition in intensive care.
Detective Sergeant Nick Bailey, who was part of the initial police response, is in a serious but stable condition.
The episode has left Britain's relations with Moscow, which were already under severe strain, at breaking point.
In an extraordinary statement in the Commons on Monday, Theresa May told MPs it was "highly likely" Russia was responsible for the poisoning.
She disclosed that the dangerous substance used in the attack was a military-grade Novichok nerve agent produced by Russia.
The PM set a deadline of midnight on Tuesday 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.
Housing Minister Dominic Raab told the BBC Radio 4 Today programme it would open up "the whole panoply of counter-measures from economic, financial, diplomatic measures", potentially including moves targeted at individuals such as visa bans and freezing financial assets.
However he downplayed the involvement of Nato in any response to Moscow, saying that Mrs May "chose her words very carefully" in her Commons statement.
He said: "The words 'unlawful use of force' are different and have a different meaning in international law from 'armed attack'."
Russian foreign minister Sergei Lavrov insisted the country was not to blame, and asked for access to samples of the poison.
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."