The UK will expel 23 Russian diplomats it has identified as "undeclared intelligence officers" in retaliation for the poisoning of a former Russian spy and his daughter on British soil.
Theresa May told the House of Commons on Wednesday that the National Security Council had agreed a range of measures, including the halting of planned diplomatic contact with Russia.
The identified diplomats, whose expulsion is being ordered under the Vienna Convention, have one week to leave.
"This will be the single biggest expulsion in 30 years and it reflects the fact that this is not the first time the Russian state has acted against our country," May said.
"Through these expulsions we will fundamentally degrade Russian intelligence capability in the UK for years to come. And if they seek to rebuild it, we will prevent them from doing so."
Other measures include:
— Russian state assets will be frozen "wherever we have the evidence that they may be used to threaten the life or property of UK nationals or residents."
— Immediate actions will be taken to "dismantle the Russian espionage network in the UK."
— Ministers and members of the royal family will boycott this summer’s World Cup in Russia.
— The UK is pushing for a full international response from the UN Security Council, which meets later on Wednesday in New York.
— The UK's foreign office has updated its travel advice for Britons visiting Russia, which now warns people to "be aware of the possibility of anti-British sentiment or harassment."
In a lengthy statement, May said there is now no doubt that the Russian state was behind the poisoning of the former Russian spy Sergei Skripal and his daughter, who remain in a critical condition after being exposed to a Russian-made military-grade nerve agent called Novachok.
On Monday, May said it was "highly likely" that Russia was behind the attack, but added that it remained possible that it had somehow lost control of the nerve agent and gave them until midnight to provide an explanation.
"It was right to provide Russia with the opportunity to provide an explanation but their response has demonstrated. complete disdain for the gravity of these events," she told the House of Commons on Wednesday.
"They have provided no credible explanation that could suggest they lost control of their nerve agent, no explanation on how it came to be used in the UK, no explanation as to why Russian has an undeclared chemical weapons programme in contravention of international law.
"Instead they have treated the use of a military-grade nerve agent in Europe with sarcasm, contempt, and defiance. So Mr Speaker, there is no alternative conclusion other than that the Russian state was culpable for the attempted murder of Mr Skripal and his daughter and for threatening the lives of other British citizens including detective sergeant Nick Bailey."
May said the attack represented an "unlawful use of force by the Russian state against the UK" which would be met with a "full and robust response."
"Many of us looked at a post-Soviet Russia with hope. We wanted a better relationship and it is tragic that President Putin has chosen to act in this way," she added.
Earlier, the Russian ambassador to the UK, Alexander Vladimirovich, said after meeting with the British Foreign Office that the UK's accusation was a "serious provocation."
He told Sky News: "I’ve had just a meeting in the Foreign Office and I said that everything what is done today by the British government is absolutely unacceptable and we consider this a provocation."
"We believe the measures which are taking by the British government is nothing to do with the situation that we have in Salisbury," Vladimirovich said. "We believe this is very serious provocation and of course we are not ready to talk in a way of the ultimatums."
In a terse statement, the Russian embassy in London said: "We consider this hostile action as totally unacceptable, unjustified and shortsighted.
"All the responsibility for the deterioration of the Russia-UK relationship lies with the current political leadership of Britain."
There were tense scenes in the Commons after Theresa May's speech, when Jeremy Corbyn, leader of the Labour party, appeared to offer equivocal support for May's position and stopped short of condemning the Russian state.
"The attack in Salisbury was an appalling act of violence. Nerve agents are abominable to be used in any war; it is utterly reckless to use them in a civilian environment," he said.
"The circumstances demand that we build an international consensus to address the use of chemical weapons. We should urge international allies to reveal without delay full details of its chemical weapons programme."
To cries of "shame" from MPs, Corbyn said it was a "matter of huge regret" that the UK's diplomatic capacity has been cut by 25%. Corbyn was criticised on Monday for what some members saw as a politicised response to a national emergency.
In response, May said: "And he said we should be trying to build a consensus. It’s clear from the conversation I’ve had with allies that we have a consensus with allies. It was clear from remarks that were made by backbenchers across the whole of this house, that there is a consensus across the back benches of this house.
"I am only sorry that the consensus does not go as far as the right honourable gentleman [Corbyn] who could have taken the opportunity, as the UK government has done, to condemn the culpability of the Russian state."
This is a developing story – check back here for updates and follow @BuzzFeedNews on Twitter.
Patrick Smith is a senior reporter for BuzzFeed News and is based in London.
Contact Patrick Smith at firstname.lastname@example.org.
Got a confidential tip? Submit it here.