collapsed on a bench in Salisbury, remain in a critical but stable condition
Theresa May is facing mounting pressure to take decisive action against whoever was behind the nerve agent attack on an ex-Russian spy and his daughter.
Former national security advisor Lord Ricketts said “firm action” was needed once responsibility was confirmed.
Foreign affairs committee chairman Tom Tugendhat said evidence was mounting against Russia, which has denied involvement in the Salisbury attack.
Sergei and Yulia Skripal remain in a critical but stable condition.
The 66-year-old retired Russian military intelligence officer and his daughter, 33, were found slumped on a bench in Salisbury, Wiltshire, on 4 March.
Mr Skripal 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”.
The prime minister is due to update MPs in the Commons on the investigation at about 16:30 GMT.
Earlier she chaired a National Security Council meeting, held to discuss the latest information with ministers and intelligence and military chiefs.
Mr Tugendhat told BBC Radio 4’s Today programme: “We are expecting to see the prime minister make an announcement soon.
“And, frankly, I would be surprised if she (Theresa May) did not point the finger at the Kremlin.”
Lord Ricketts told Today he had “little doubt that this had come from Russia”.
“The really difficult judgement is who authorised it and why – was it some sort of rogue element or was it centrally authorised? I suppose that does have some impact on the kind of reactions that we need,” he said.
However, Dmitry Peskov, press secretary to Russian President Vladimir Putin, said: “This incident happened on British soil and it doesn’t have anything to do with the Russian Federation whatsoever, let alone the president.”
The prime minister’s official spokesman said: “If we get to a position when we are able to attribute this attack then we will do so and the government will deliver an appropriate response.”
On Sunday, up to 500 Salisbury pub-goers and diners were told to wash their possessions as a precaution.
Trace amounts of the substance used to poison Sergei and Yulia Skripal were found on and around a table where they had eaten in Zizzi. The table has since been destroyed.
Traces were also found at the Mill pub in Salisbury.
Prof Dame Sally Davies, the chief medical officer for England, said the advice was a “belt and braces” measure, adding: “I am confident none of these customers or staff will have suffered harm.”
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
Some Salisbury residents have voiced concern that the advice was not given sooner.
Steve Cooper, who was at the Mill pub with his wife, 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 added.
Salisbury City Council leader Matthew Dean believes the advice to wash clothes was timely enough.
He told the BBC: “I think what I am very confident about is that consistently the advice has been that this is a very, very precautionary approach and that they are advising that people wash their clothes because they don’t want people to come into prolonged contact,” he said.
Det Sgt Nick Bailey, who fell ill attending Mr Skripal and his daughter, 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 his wife and son are buried are also being examined.
Wiltshire Police have not said how long those crime scenes will remain closed to the public.
Nick Bailey remains in hospital
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.