The prime minister is to meet the National Security Council to hear the latest intelligence on the nerve agent attack on the former Russian military intelligence officer and his daughter.
Chaired by Theresa May, the council will discuss if there is sufficient evidence of who was behind the attack.
On Sunday up to 500 Salisbury pub-goers and diners were told to wash their possessions as a precaution.
The victims, Sergei and Yulia Skripal are in a critical but stable condition.
Trace amounts of the substance used to poison the pair were found on and around a table where they ate on 4 March.
Monday’s meeting of the security council will bring together senior ministers, as well as intelligence and military chiefs.
They will consider whether there is enough evidence to take action, and whether it is the right time to do so, said the BBC’s home affairs correspondent Daniel Sandford.
As well as in Zizzi’s restaurant, traces of the nerve agent were also found at the Mill pub in Salisbury.
Prof Dame Sally Davies, the chief medical officer for England, said the risk of harm to fellow diners was “low”.
The advice to wash possessions applies to anyone in either venue between 13:30 GMT on Sunday 4 March to closing time on Monday. People have been advised:
- Clothes should be washed, ideally in a washing machine
- Clothes which cannot be washed, for example if they need dry cleaning, should be double bagged in plastic until further notice
- Mobile phones, handbags and other electronic items should be wiped with baby wipes, which should be bagged in plastic and put in the bin
- Other items such as jewellery and glasses should be washed with warm water and detergent
- Hands should be washed after the handling of any items suspected of being contaminated
Dame Sally said the advice was a “belt and braces” measure, adding: “I am confident none of these customers or staff will have suffered harm.”
Some Salisbury residents were concerned the advice was not given sooner.
Steve Cooper, who was at the Mill pub with his wife last Sunday afternoon, told the BBC he was outraged.
He said he would now wash his shoes, watch and phone but feared using baby wipes would not get rid of a nerve agent.
“I’d like to know what are the long-term implications to me and my wife,” he said.
Dame Sally said it had been a “painstaking process” but no harm had been caused by the wait.
Mr Skripal, 66, and his daughter Yulia, 33, were found slumped on a bench in the city.
Det Sgt Nick Bailey, who fell ill attending the pair, remains seriously ill in hospital but has been talking to his family.
The pub and restaurant are two of five sites in Salisbury focused on by investigators.
Mr Skripal’s home and the cemetery where Mr Skripal’s wife and son are buried are also being examined.
At a press conference on Sunday, Chief Constable Kier Pritchard of Wiltshire Police said he was “unable to clarify” how long those crime scenes would remain closed to the public.
Military personnel continued to assist police over the weekend, and removed vehicles including an ambulance.
More than 250 counter terrorism police are involved in the investigation, which so far has yielded 200 pieces of evidence and more than 240 witnesses.
Mr Skripal, a retired Russian military intelligence officer, was convicted by the Russian government of passing secrets to MI6 in 2004, but given refuge in the UK in 2010 as part of a “spy swap”.
Russia has denied any involvement n the attack.
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