Theresa May is preparing to chair a meeting of the national security council after the midnight deadline she set Moscow over the spy poisoning case passed.
Russia said on Tuesday it would ignore demands for an explanation over how a nerve agent was deployed in Salisbury unless Britain agreed to send it samples of the poison.
The Federation’s embassy in the UK fired off a salvo of tweets warning the threat of sanctions would “meet with a response”.
Counter-terrorism police, meanwhile, launched an investigation into the death of a Russian exile who was a close friend of Vladimir Putin critic Boris Berezovsky.
Scotland Yard said a man in his 60s was found at a home in Clarence Avenue, New Malden, south-west London on Monday and that the cause of his death is unexplained – but there was “no evidence to suggest a link to the incident in Salisbury”.
Mrs May has warned she will set out a “full range” of measures to be taken in response if there is no credible response from the Kremlin about the attack on Sergei Skripal and his daughter Yulia.
She will chair a meeting on Wednesday morning of senior intelligence officials and ministers to discuss developments in the case.
The premier is likely to make a statement in the House of Commons to update MPs after Prime Minister’s Questions.
Britain’s options are being weighed up by the Government – with a cyber counter-strike said to be among the possible measures being considered, along with economic, financial and diplomatic action.
In a series of tweets, the Russian Embassy said: “Moscow will not respond to London’s ultimatum until it receives samples of the chemical substance to which the UK investigators are referring.”
“The incident appears to be yet another crooked attempt by the UK authorities to discredit Russia,” it added.
The embassy said those calling for the expulsion of Russian diplomats “don’t care about Global Britain and its diplomats in Moscow”.
World leaders, including Donald Trump and Angela Merkel, voiced their support for the UK as the deadline approached.
Mrs May said on Monday the Government had concluded it is “highly likely” Russia was responsible for the attack which left ex-spy Mr Skripal and his daughter in a critical condition in hospital.
She demanded that Moscow account for how a Russian-produced nerve agent could have been deployed in Salisbury and vowed to set out measures Britain would adopt if no credible response was received by the end of Tuesday.
President Trump told Mrs May in a phone call the US is “with the UK all the way” and German Chancellor Angela Merkel said she stood in “full solidarity” with Britain, according to Downing Street.
Police said investigations could take several weeks as they carry out a “painstaking” operation to identify how the Novichok nerve agent was used to poison Mr Skripal on British soil.
The former double agent, 66, and his daughter, 33, were found slumped on a bench in Salisbury, Wiltshire, on March 4.
Police and MI5 are to look into allegations that a string of other deaths on UK soil may be linked to Russia.