The Home Secretary is to make a Commons statement on the Salisbury poisoning mystery amid calls for a police investigation into 14 suspected spying-related deaths in recent years.
Amber Rudd will report to MPs on the dramatic disclosure by Scotland Yard that Sergei Skripal and his daughter Yulia were poisoned by a deadly nerve agent and targeted in an attempt to murder them.
Anti-terror chiefs have also revealed that a police officer who was first on the scene when the former Russian double agent and his daughter were discovered is now also fighting for his life in hospital.
In a tweet on the eve of her statement, the Home Secretary said: "I want to express my heartfelt thanks to the emergency services for their bravery and professionalism in continuing to deal with the incident in Salisbury.
"My thoughts are with all those affected, including the police officer who is being treated in hospital."
Earlier, Foreign Secretary Boris Johnson said the events in Salisbury were "very troubling".
Speaking at a news conference at the Foreign Office alongside the visiting Saudi Arabian foreign minister, he was asked if the latest evidence meant it was now time to call in the Russian ambassador.
He replied: "If this does turn out to be in any way the result of hostile activity by another government, or directed, led, by another government, then the people of this country can be absolutely sure that the UK will respond robustly."
As the poisoning crisis escalates with more alarming disclosures, the Home Secretary will face renewed demands from Yvette Cooper, who chairs the Home Affairs Select Committee, and other MPs to order an inquiry into earlier unexplained deaths.
In a letter to the Home Secretary, Ms Cooper said 14 deaths that have not been treated as suspicious by the UK police have reportedly been identified by United States intelligence sources as potentially connected to the Russian state.
Senior Home Office sources are playing down speculation that the Home Secretary will use her Commons statement to announce an inquiry, saying it is "too soon", though they acknowledge it is under serious consideration.
Meanwhile, Mr Skripal, his daughter and a police officer who tried to help them are all in a coma in intensive care, Sky sources said.
Dame Sally Davies, chief medical officer for England, said there was a "low risk to the public" and the sites Mr Skripal and his daughter visited have been secured.
Mr Skripal was convicted in Russia of spying for MI6 in 2006, and later given refuge in the UK as part of a spy swap.
Yulia currently works for Pepsico in Moscow, according to her Facebook account, after moving to Britain for a few years in 2010.
A friend of Yulia's, who wanted to remain anonymous, told Sky News: "We were in her house many times and she was really open for parties, meetings, to go somewhere.
"All the time she did not show what was bothering her, no, she will not show you."