Russia says it will "soon" expel British diplomats in response to Theresa May's announcement that 23 "undeclared intelligence officers" posing as Russian diplomats would be ejected from the UK.
Foreign Minister Sergei Lavrov said it was unacceptable for Britain to blame Russia for the poisoning of the ex-Russian spy Sergei Skripal in Salisbury.
Asked if Moscow was ready to kick out British diplomats in retaliation, Mr Lavrov said "Absolutely. Soon," according to the RIA news agency.
And in an apparent criticism of the way Ms May unveiled a series of measures against Russia in Parliament on Wednesday, Mr Lavrov said Russia would first inform the British government first about an retaliatory action before revealing them to the wider public.
Mr Skripal, 66, and his daughter Yulia, 33, were found unconscious on 4 March in the centre of Salisbury and remain critically ill in hospital. Ms May says tests showed a Soviet-era nerve agent known as Novichok was used in the attack, and that Russia was therefore "highly likely" to be responsible.
In comments on Thursday morning, the UK Foreign Secretary Boris Johnson expanded on Ms May's statement, saying the evidence of Russian guilt was "overwhelming" because only Moscow had access to the poison and also a motive for harming Mr Skripal.
Ratcheting up the rhetoric, he told the BBC: "There is something in the kind of smug, sarcastic response that we're heard from the Russians that to me betokens their fundamental guilt.
"They want to simultaneously deny it and yet at the same time to glory in it."
Mr Johnson said the attack was a way for Putin to send a message to anyone considering taking a stand against it that 'You do that, you are going to die'.
Russia has taken umbrage to the way Ms May announced the UK's position, first giving Moscow a 24-hour deadline in which to explain how the poison came to be used in southern England.
On Wednesday evening, Moscow’s ambassador to the UN said Russia does not “speak the language of ultimatums” and will not “be spoken to in that language either”.
“Their response demonstrated complete disdain for the gravity of these events,” Ms May said in her statement to Parliament.
“They have treated the use of a military-grade nerve agent in Europe with sarcasm, contempt and defiance.”
France, which on Wednesday had said it wanted proof of Russian involvement before deciding whether to take action against Russia, appeared to change its position on Thursday, saying it agreed with the assessment of its NATO ally Britain.
"France agrees with the United Kingdom that there is no other plausible explanation (than Russian involvement) and reiterates its solidarity with its ally," President Emmanuel Macron's office said.
The US Ambassador to the UN, Nikki Haley, said on Wednesday that Washington believed Moscow was responsible for the attack, adding it was a crime worthy of UN Security Council action.
Any effective Security Council action seems highly unlikely, however, given that Russia, like Britain and the United States, is a permanent, veto-wielding member of the body.
Russia has repeatedly said the UK is refusing to provide a sample of the nerve agent used in Salisbury, initially saying it would not respond to Ms May's ultimatum until this condition was met.
Mr Johnson said the UK would send a sample of the nerve agent to the Organisation for the Prevention of Chemical Weapons for independent assessment.
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