Russia is to expel 23 British diplomats amid the ongoing 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 diplomats 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Russia has also withdrawn permission for Britain to open a general consulate in St Petersburg, and says it will be closing the British Council in Russia.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 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."
Nerve agent attack
Russia has continued to deny any responsibility for the attack.
The UK's Foreign Secretary Boris Johnson has said it is "overwhelmingly likely" that Russian President Vladimir Putin signed off on 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.
The victims of the Salisbury attack, meanwhile, remain in a critical but stable condition in hospital, while a police officer also remains in a serious condition.