Russia will refuse to meet Theresa May's midnight deadline unless Britain agrees to send Moscow samples of the nerve agent used to poison Sergei Skripal.
The country's embassy in the UK fired off a salvo of seven tweets in which it said Britain must comply with the Chemical Weapons Convention for a joint investigation and warned the threat of sanctions would "meet with a response".
It comes as President Donald Trump told Mrs May in a phone call the US is "with the UK all the way", according to Downing Street.
Mrs May said the Government had concluded it is "highly likely" Russia was responsible for the attack which left ex-spy Mr Skripal and his daughter Yulia 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 will adopt if no credible response is received by the end of Tuesday.
Home Secretary Amber Rudd earlier said Russia had "started responding" but the embassy appeared to suggest this amounted to little more than informing the Foreign Office of its demands and reiterating it was not involved.
Britain's ambassador to Russia was summoned by Moscow and told by first deputy foreign minister Vladimir Titov that the Kremlin "strongly protested" the accusations, the embassy said.