Russia is demanding access to samples of the nerve agent that poisoned ex-spy Sergei Skripal and his daughter Yulia in Salisbury, Foreign Minister Sergei Lavrov has said.
The Russian government has been given until midnight on Tuesday to explain its involvement in the poison attack, which Theresa May said was "highly likely" to have been ordered by Moscow.
Unless Vladimir Putin's government can provide a compelling answer, the UK will conclude that Russia carried out an "unlawful use of force" on British soil, the Prime Minister said.
Investigators have identified the nerve agent used in the attack as 'Novichok', a family of chemical weapons developed by the Soviet Union in the 1970s and 80s which is among the most deadly of its kind, and designed to be easy to transport and hard to detect.
READ MORE: UK could impose ‘extensive’ sanctions on Russia for poison attack
In an interview, Foreign Secretary Boris Johnson said Russia had to "explain how it came to be that Novichok was used on the streets of Wiltshire".
"If they can come up with a convincing explanation then obviously we will want to see full disclosure of that to the Organisation for the Prohibition of Chemical Weapons in the Hague," Mr Johnson said. "If not, then clearly we will want to be announcing the UK response and that will come tomorrow."
After the White House declined to repeat the Prime Minister's comments about Russian involvement in the attack, the US Secretary of State Rex Tillerson said he had "full confidence in the UK’s investigation and its assessment that Russia was likely responsible for the nerve agent attack".
Mrs May also spoke to French President Emmanuel Macron on Monday night and secured his backing.
"I’ve been very encouraged so far by the strength of the support that we are getting," Mr Johnson added. "I talked to Sigmar Gabriel my German counterpart, and from Washington where Rex Tillerson last night made it absolutely clear that he sees this as part of a pattern of disruptive behaviour, increasingly disruptive behaviour, malign behaviour by Russia, the reckless use of chemical weapons, the support for the reckless use of chemical weapons which stretches from Syria now to the streets of Salisbury.
"I’ve been encouraged by the willingness of our friends to show support and solidarity."
In a separate interview for foreign broadcasters, Mr Johnson said: "We don’t want to demonise either Russia or the Russian people for whom obviously we have a very high regard.
"Russia is a great country. It is a great pity therefore that the Russian regime seems to be moving in this dangerous and disruptive direction."
Mrs May will update the House of Commons again on Wednesday, with the Prime Minister expected to announce the expulsion of a large number of Russian diplomats, as well as new sanctions and restrictions on senior Russian officials.
MPs from across the House have also called for a ban on the state-funded broadcaster Russia Today, putting pressure on the former Scottish First Minister Alex Salmond cancel his programme on the station.
Speaking on the BBC's Today programme, the housing minister, said Mrs May had used her words "very carefully" in Monday's Commons statement.
"The words ‘unlawful use of force’ have a different meaning in international law from an armed attack," Mr Raab said. "The range [of measures] would I believe extend to and include: diplomatic measures, financial measures, economic measures and issues around visa bans and things like that."