The Prime Minister has said it is "highly likely" Russia was behind the poisoning of former double agent Sergei Skripal and his daughter Yulia last weekend.
Speaking just hours after chairing a second National Security Council meeting, Theresa May told MPs the pair had been "poisoned by military grade agent, of a type developed by Russia".
She revealed the name of the agent was Novichok, one that has been previously produced by Russia.
Based on that information, and Russia's record of state sponsored assassinations, as well as "our assessment that Russia views defectors as legitimate targets for assassinations", the government has concluded it is "highly likely Russia was responsible for this act against Sergei and Yulia Skripal," she said.
The Russian ambassador has been called into the Foreign Office by Boris Johnson to explain whether it was a "direct act by the Russian state against our country" or if Russia had "lost control" of the agent and allowed it to fall into a third party's hands.
"If there is no credible response from Russia, the government will conclude it amounts to unlawful use of force by Russian state against the UK," she said.
Russia has until midnight tomorrow to respond to Johnson, and May will set out her position to the Commons on Wednesday.
"This attempted murder using a weapons-grade nerve agent in a British town was not just a crime against the Skripals. It was an indiscriminate and reckless act against the United Kingdom, putting the lives of innocent civilians at risk.
"We will not tolerate such a brazen attempt to murder civilians on our soil."
Labour leader Jeremy Corbyn used the opportunity to score a political point, asking the Prime Minister about Conservative donations from Russian oligarchs. But his question was met with jeers and shouts of "shame", and MPs from his own party - including Yvette Cooper and Chris Leslie - said it was time to put aside party politics.
However, unity did not last long.
Conservative Tom Tugendhat, who chairs the foreign affairs select committee, told MPs: "This, if not an act of war, was certainly a warlike act". He cited Russian attacks on allies, attempts to kill leaders of other countries and its ongoing backing of Syria's Bashar al-Assad.
"And the honourable gentleman opposite stays silent" he added, pointing at Corbyn.
Earlier in the day Tugendhat had told the BBC the case looked "awfully like state-sponsored attempted murder".
Skripal and his daughter Yulia have been in intensive care for more than a week, having been found unconscious on a bench in Salisbury last Sunday. The pair had come into contact with a nerve agent that experts have reportedly ruled "beyond a reasonable doubt" to have come from Russia.
A police officer is also still being treated in hospital, in a "serious but stable condition", the Prime Minister said today, as she paid tribute to him in the Chamber.
Russia has so far rejected any suggestion that it was involved in the attempted attack and Russian state TV this weekend suggested Britain itself had poisoned Skripal to upset this summer's World Cup which Russia is hosting.
“They tried to pin the blame on Russia, but if you think it through the poisoning of the GRU (military intelligence) colonel was only advantageous to the British,” one presenter said, adding "Skripal is cheap expendable material".
Skripal is a retired Russian military intelligence officer who was jailed for 13 years by Russia in 2006 for spying for Britain.
He was convicted of passing the identities of Russian intelligence agents working undercover in Europe to the UK's Secret Intelligence Service, MI6.
The 66-year-old double agent was granted refuge in the UK following a "spy swap" between the US and Russia in 2010.
In 2010, Putin made it clear what would happen to those found guilty. "Traitors will kick the bucket, trust me,” he said at the time.
“These people betrayed their friends; their brothers in arms. Whatever they got in exchange — those 30 pieces of silver they were given — they will choke on them.”