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.
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."