5.17pm: Following a meeting of the British Government's Cobra committee, Home Secretary Amber Rudd said there were more than 250 counter-terrorism police involved in the investigation which was proceeding with "speed and professionalism".
Police investigating the Salisbury nerve agent attack have identified more than 200 witnesses and 240 pieces of evidence, Ms Rudd said.
Following the Cobra meeting, Ms Rudd said there were more than 250 officers from eight out of 11 of the UK's counter-terrorism units involved in the investigation.
"This is a serious, substantial investigation. We need to give the police and the investigators the space to get on with that," she said.
"I want to stress that they are proceeding with speed and professionalism. We are putting in enormous resources to ensure that they have all the support that they need to do that."
7.50am: Emergency Cobra meeting called over UK nerve agent attack
Investigators have been summoned for emergency talks on the crisis in Salisbury, where the fallout from a suspected nerve agent attack continues to widen.
UK Home Secretary Amber Rudd will chair a meeting of the Government's Cobra committee at 3pm on Saturday to receive updates on the police case, Downing Street said.
Russian double agent Sergei Skripal and his daughter Yulia are still fighting for their lives after being exposed to a toxic substance in the Wiltshire city last Sunday.
Detective Sergeant Nick Bailey, who was part of the initial response by authorities, is also in a serious condition.
Around 180 troops, including Royal Marines, RAF Regiment troops and chemical warfare specialists, are understood to have been deployed in the city on Friday after Scotland Yard requested specialist help.
A Ministry of Defence spokesman said armed forces personnel will return to Salisbury in similar numbers on Saturday as the probe and cleanup operation continues.
Police turned their attention on Friday to the cemetery where the 66-year-old Russian's wife and son were laid to rest.
Officers in hazmat suits were seen placing a blue forensic tent over his son's memorial stone before appearing to stuff items in several yellow barrels.
The grave of Mr Skripal's wife Liudmila, who was buried in 2012, and the memorial stone of his son Alexander, who was cremated last year, were cordoned off at the London Road cemetery.Apparent fears of chemical contamination have also seen Mr Skripal's home cordoned off while detectives attempt to pin down the origins of the substance used to incapacitate him.
A short distance away, a convoy of military vehicles rolled into the car park at Salisbury District Hospital to recover a police car.
Mr Skripal and Yulia, 33, are still in a "very serious" condition five days after they were discovered slumped on a bench in the city centre.
Suspicion is mounting that Russia carried out the attempt on their lives as an act of revenge against the former intelligence officer, who was convicted in 2006 of selling state secrets to MI6.
He was later released as part of a spy swap with the US.
The Kremlin denies responsibility and British ministers have urged caution over apportioning blame until the facts become clear.
Defence minister Tobias Ellwood told the Press Association that the military's presence reflected the "seriousness" of the situation, adding: "We mustn't get ahead of ourselves but we must have a robust response and it's something that we'll be discussing with our Nato partners and with the forthcoming summit in Brussels in July.
"Some big questions arise as to how do you stand up to a clandestine and sinister attack deliberately done to play havoc in our society?"
Meanwhile, Salisbury's MP John Glen, also Government minister, attempted to reassure his constituents that a "whole range of tools are at our disposal" once it is established who was behind the incident.
"The Government will not be acting precipitously in response to what has happened. Now is the time for cool heads and a rational examination of the facts," he said.
"Once these are established, then and only then, will an appropriate and proportionate course of action be taken. A whole range of tools are at our disposal depending on who has perpetrated this act, including a number of financial and economic levers."
Mrs Rudd had earlier visited Salisbury and the hospital where DS Bailey is receiving treatment.
Lord Blair, a former Met Commissioner, suggested on the BBC's Today programme the seriously ill detective had visited Mr Skripal's home.
He said: "There obviously are some indications the officer, and I'm very sorry that he has been injured, has actually been to the house, whereas there was a doctor who looked after the patients in the open who hasn't been affected at all.
"So there may be some clues floating around in here."
Police said 21 people had been seen for medical treatment since the incident.
The figure includes members of the public and emergency staff, some of whom have had blood tests as well as receiving support and advice.
The attack is being treated as attempted murder.
- Press Association