Russia is to expel 23 British diplomats following the row over the poisoning of ex-spy Sergei Skripal and his daughter Yulia in Salisbury.
The move follows the UK's expulsion of 23 Russian ambassadors earlier this week, after Russia failed to respond to a deadline set by Theresa May for Moscow to explain whether it was behind the attack.
The British ambassador to Russia Laurie Bristow was summoned for talks with the Russian Foreign Ministry on Saturday.
Moscow has said the diplomats must leave Russia within one week.
Russia has also withdrawn permission for Britain to open a general consulate in St Petersburg and says it will be closing the British Council in Russia.
Additionally, the Russian foreign ministry has said it reserves the right to take other measures against Britain in the event of further hostile steps from London.
Leaving the meeting, Mr Bristow said: "This crisis has arisen as a result of an appalling attack in the UK, the attempted murder of two people using a chemical weapon developed in Russia and not declared by Russia to the Organisation for the Prohibition of Chemical Weapons (OPCW) as Russia is obliged to do under the Chemical Weapons Act.
"The Prime Minister set out in Parliament a number of measures which we took to defend ourselves against this type of attack. We gave Russia the opportunity to explain how the material got to Salisbury and we asked Russia to declare that material to the OPCW. Russia did neither, therefore we announced certain steps."
Earlier in the week, Mr Bristow had met with Russian deputy foreign minister Vladimir Titov, however there were conflicting reports over the reason for the meeting
Russian news agencies reported that it was to discuss the poisoning, while the UK embassy in Russia said it was a pre-planned meeting.
However, Mr Bristow confirmed that the attack on the former Russian spy and his daughter was discussed in the meeting, saying: "I reiterated the points made by Prime Minister May that we expect by the end of today an account from the Russian state as to how this material came to be used in Salisbury."
The midnight deadline then passed without comment from Russia, prompting the UK expulsion of the Russian diplomats.
Russia has continued to deny possessing the deadly nerve agent - novichok - confirmed to have been used in the attack.
When asked by Sky News whether Russia had destroyed all its chemical weapons in line with the internationally recognised Chemical Weapons Convention, Russian diplomat, Alexander Shulgin said: "Absolutely. No question."
Foreign Secretary Boris Johnson has said it is "overwhelmingly likely" that Russian President Vladimir Putin signed off the nerve agent attack.
In response, Russian foreign minister Sergei Lavrov called Mr Johnson's claims "shocking and unforgivable".
Mr Lavrov had earlier said that Moscow had "stopped paying attention" to the poisoning claims, and called UK's refusal to work with Russia a "violation of international agreements".
Today's meeting is due to take place later this morning, with UK ambassador expulsions expected to follow.
Source: Sky News