Russia said Saturday it would expel 23 British diplomats and close a British consulate following London's "provocative" measures over the poisoning of a double agent that has triggered an escalating war of words.
And it also said it would halt the activities of the British Council in Russia in a tough series of retaliatory measures announced after summoning British ambassador Laurie Bristow.
The Russian response was announced on the eve of a presidential election which is expected to hand Vladimir Putin a fourth term in the Kremlin, but which comes as the country appears increasingly isolated.
The crisis erupted after Sergei Skripal and his daughter Yulia were exposed to a Soviet-designed nerve agent on March 4 in the English city of Salisbury, leaving them in critical condition.
London has blamed Moscow and on Friday, even directly implicated Putin in the attack, prompting the Kremlin's fury.
"Twenty three diplomatic staff at the British embassy in Moscow are declared persona non grata and to be expelled within a week," a foreign ministry statement said.
It said this was a response to Britain's "provocative actions" and "baseless accusations over the incident in Salisbury on March 4".
- 'Harsh, but deserved' -
Russia also said that it was withdrawing permission for Britain to operate its consulate in the northwestern city of Saint Petersburg, citing a "disparity" in the number of diplomatic missions held by the two countries.
And it said it had halted the activities of the British Council, Britain's international organisation for cultural relations and educational opportunities, across the country.
"Due to the unregulated status of the British Council in Russia, its activity is halted," the foreign ministry said.
"The measures are more harsh, but the British deserved them. And I don't rule out that something else could follow," first deputy head of the Russian Senate's foreign affairs committee Vladimir Dzhabarov told Interfax news agency.
A representative for the British Embassy in Moscow told AFP it had no immediate statement in response to the moves but would release one on social media.
The ministry said it had also warned Britain that "if further unfriendly actions are taken towards Russia, the Russian side retains the right to take other answering measures."
Earlier this week, British Prime Minister Theresa May announced the expulsion of 23 Russian diplomats and suspension of high-level contacts over the nerve agent attack.
And she also warned more measures could follow, noting that the US-led NATO alliance and the UN Security Council had discussed the attack.
- First nerve attack since WW2 -
Skripal had taken his daughter, who was on a visit from Moscow, out for lunch in Salisbury before they both collapsed on a bench.
The incident revived memories of the fate of Alexander Litvinenko, a Russian dissident who died of Polonium radiation poisoning in London in 2006 in an attack that Britain also blamed on Russia.
Russia insists it had no motive to target Skripal with what Britain says was a highly-potent Soviet-designed nerve agent called Novichok, in the first such attack in Europe since World War II.
On Friday, Kremlin spokesman Dmitry Peskov said statements by British Foreign Secretary Boris Johnson tying Putin to the attack were "shocking and unforgivable".
Putin himself has yet to make a public comment on the attack, aside from one remark to a BBC reporter earlier this week in which he said: "Sort things out from your side and then we will discuss this with you."
Western leaders have strongly backed Britain's response.
EU leaders are to discuss the incident at a Brussels summit next week and it is also on the agenda for talks on Monday between Johnson, his EU counterparts and NATO Secretary General Jens Stoltenberg.
- Suspicious deaths -
Following the Salisbury incident, the British government has also pledged to re-examine 14 deaths on UK soil following a report that they could have been carried out by Moscow or the Russian mafia.
On Friday, police said they were treating the March 12 death of exiled businessman Nikolai Glushkov as murder after a post-mortem found he died from "compression to the neck".
The murder is not believed to be linked to the attack on Skripal and his daughter.
Russia has also said it is opening a probe into the murder of Glushkov, who was found dead at his London home.
Glushkov was an associate of the late Kremlin opponent Boris Berezovsky, a one-time Putin supporter who then turned against him. Berezovsky was found hanged in a bathroom at his home outside London in 2013.