Russia is kicking out 23 British diplomats - many believed to be intelligence officers - as the row intensifies over the poisoning of ex-spy Sergei Skripal and his daughter Yulia in Salisbury.
The move follows the UK's expulsion of 23 Russian diplomats - thought to be intelligence agents - earlier this week, after Moscow failed to respond to a deadline set by Theresa May for the Kremlin to explain whether it was behind the nerve agent attack.
The British ambassador Laurie Bristow was summoned for talks with the Russian foreign ministry on Saturday, and Moscow has said the UK diplomats must leave the country within one week.
Sky's Defence Correspondent Alistair Bunkall tweeted: "I understand the 23 expelled British diplomats are intelligence officers. Looks like Russia has gone for exact tit-for-tat."
Russia has also withdrawn permission for Britain to open a general consulate in St Petersburg and says it will be closing the British Council in Russia.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.
Following 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 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."
Mr Bristow reiterated that the UK has no dispute with the Russian people, but said "we will always do what is necessary to defend ourselves, our allies and our values against an attack of this sort which is not only an attack on the UK but also on the international rules based systems on which all countries - including Russia - depend for their safety".
Earlier in the week, Mr Bristow 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 4 March 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".
Russian elections are due to take place on Sunday, with Mr Putin seeking a fourth term in office after being in power for 18 years. He is widely predicted to win.