Russia has been given a midnight deadline by British officials to explain the nerve agent attack on a former Russian spy and his daughter.
Sergei (66) and Yulia (33) Skripal remain in a critical condition after being found unconscious on a bench in Salisbury more than a week ago.
Officials have determined they had been subjected to a nerve agent in what has been described as attempted murder.
Yesterday, British Prime Minister Theresa May said it was 'highly likely' Russia was behind the attack - a claim Russian officials have denied.
She stated that the pair had been hit with Novichok - referring to a string of chemical weapons developed in Russia from the 1970s onwards
She told MPs: "Based on the positive identification of this chemical agent by world-leading experts at the Defence Science and Technology Laboratory at Porton Down; our knowledge that Russia has previously produced this agent and would still be capable of doing so; Russia's record of conducting state-sponsored assassinations; and our assessment that Russia views some defectors as legitimate targets for assassinations; the Government has concluded that it is highly likely that Russia was responsible for the act against Sergei and Yulia Skripal."
She suggested: "Either this was a direct act by the Russian State against our country, or the Russian government lost control of this potentially catastrophically damaging nerve agent and allowed it to get into the hands of others."
She noted that the British Foreign Secretary Boris Johnson had spoken to the Russian Ambassador to the UK, requesting "full and complete disclosure of the Novichok programme to the Organisation for the Prohibition of Chemical Weapons" by the end of today.
US Secretary of State Rex Tillerson has said his country agrees Russia was "likely responsible" for the attack, saying it "certainly will trigger a response."
In a statement, he said: "There is never a justification for this type of attack – the attempted murder of a private citizen on the soil of a sovereign nation – and we are outraged that Russia appears to have again engaged in such behavior.
"From Ukraine to Syria – and now the UK – Russia continues to be an irresponsible force of instability in the world, acting with open disregard for the sovereignty of other states and the life of their citizens."
He added: "We agree that those responsible – both those who committed the crime and those who ordered it – must face appropriately serious consequences."
Former military intelligence officer Sergei Skripal was convicted in Russia in 2006 of passing state secrets to Britain, before later being given refuge in the UK as part of a spy swap.