Theresa May has expelled 23 Russian diplomats and announced that no Royal or government figures will attend this summer's World Cup in Russia.
The Prime Minister announced the measures today (March 14) as retaliation for the poisoning of Sergei Skripal, which she has pinned squarely at the door of Russian leader Vladimir Putin.
May had given the Russian President a deadline of midnight last night to provide a "credible" response to allegations of its involvement in the Salisbury spy poisoning. However the deadline passed without a Russian response.
Addressing the Commons today, May said that all bilateral diplomatic relations with Russia will be severed, and the expelled diplomats are to leave the UK within a week. She will also push for a "robust" international response at the UN.
"Many of us looked at a post-Soviet Russia with hope," May said. "We wanted a better relationship, and it is tragic that President Putin has chosen to act in this way."
A spokesperson for Putin has insisted that Moscow had "no connection" with the attack on double-agent Sergei Skripal, who remains in hospital with daughter Yulia after being discovered slumped on a park bench in the Wiltshire city on March 4.
Kremlin spokesman Dmitry Peskov told reporters in Moscow that Russia "won't accept absolutely unfounded accusations against it, which are not substantiated by any evidence, and won't accept the language of ultimatum".
He repeated Russia's offer to co-operate in the investigation but said Moscow had received no "mutual readiness" from the UK, which has declined to share samples of the Novichok nerve agent believed to have been used.
According to Russian news agency Tass, Mr Peskov said: "We hope that common sense will prevail and that other countries will at least stop to think if there is any proof or there is none, and if reproaches against Moscow have grounds to rely on."
Counter-terrorism police, meanwhile, launched an investigation into the death of Russian exile Nikolai Glushkov, who was a close friend of Vladimir Putin critic Boris Berezovsky.
Scotland Yard said a man in his 60s was found at a home in Clarence Avenue, New Malden, south-west London, on Monday and that the cause of his death is unexplained - but there was "no evidence to suggest a link to the incident in Salisbury".
Police and MI5 are to look into allegations that a string of other deaths on UK soil may be linked to Russia.
May said on Monday the Government had concluded it is "highly likely" Russia was responsible for the attack which left Skripal, 66, and his 33-year-old daughter in a critical condition in hospital.
She warned she would set out a "full range" of measures to be taken in response if there was no credible response from the Kremlin by midnight on Tuesday.
World leaders, including Donald Trump and Angela Merkel, have voiced their support for the UK.
Britain's options are being weighed up by the Government - with a cyber counter-strike said to be among the possible measures being considered, along with economic, financial and diplomatic action.
Russia's embassy in the UK last night fired off a salvo of tweets warning the threat of sanctions would be met "with a response".
The former director of the Government's GCHQ eavesdropping centre, Robert Hannigan, said the events in Salisbury were "part of a pattern where a modern nation has chosen to step outside the rules that govern behaviour of civilised countries".
Hannigan told BBC Radio 4's Today programme the response should include "the expulsion of diplomats on a scale we probably haven't seen since the Cold War" but also "hitting the economic targets" including those who do business in London.
But he warned against a large-scale cyber attack against Russia, which he said would "play to Putin's narrative and probably wouldn't achieve much except damage all around".
Shadow chancellor John McDonnell said sanctions should be focused on the imposition of "Magnitsky" measures targeting the assets of wealthy Russians in the UK.
He rejected calls for the England football team to be pulled out of the World Cup, telling BBC Radio 4's Today programme: "I'm more in favour of our fans going and mixing with Russian people... If it's just England withdrawing it wouldn't be effective."