Boris Johnson has accused Russia of “stockpiling” the deadly nerve agent believed to have poisoned former spy Sergei Skripal and his daughter Yulia.
The Foreign Secretary said Moscow had been creating the deadly novichok over the last decade – and investigating how to use chemicals like it in an assassination.
Mr Johnson revealed: “We actually have evidence within the last 10 years that Russia has not only been investigating the delivery of nerve agents for the purposes of assassination, but has also been creating and stockpiling novichok.”
His comments came ahead of the arrival in London of experts from the chemical weapons watchdog on Monday.
Scientists from the Organisation for the Prohibition of Chemical Weapons will take samples and send them to “reputable international laboratories”.
The results are expected to take at least two weeks.
Mr Johnson announced the move on Sunday, marking the latest step by the UK to build an international coalition against Russia following the poisoning of the Skripals two weeks ago in Salisbury.
The Foreign Office said the move “reflects the UK’s commitment to fully complying with the obligations of the Chemical Weapons Convention”.
Mr Johnson also backed a suggestion that Russia had told a “direct lie” about its chemical weapons stores.
“That is exactly the tactics we’ve come to expect from Russia over the last few years,” he told the BBC’s Andrew Marr Show.
Suggestions by Russia’s ambassador to the EU that the poisoning could have been carried out by Britain were dismissed as “satirical”.
“The UK is, as it were, in the Kremlin’s crosshairs,” Mr Johnson warned.
Sky sources have also said authorities are investigating whether the nerve agent used in the attack was administered via the ventilation system in Russian Mr Skripal’s car.
It came as Labour hardened its stance against Russia.
Shadow chancellor John McDonnell said he agreed “completely” with Mrs May that Moscow either carried out the 4 March attack or lost control of a nerve agent.
Hitting out at Russian President Vladimir Putin, Mr McDonnell told ITV’s Peston on Sunday: “He is responsible whichever way you look at it. All the evidence points to him.”
But Tory party chair Brandon Lewis has reiterated Tory criticism of the Labour leadership.
He told Sky’s Sunday with Niall Patterson it was “inappropriate” for leader Jeremy Corbyn to have raised the issue of Russian donors to the Conservatives.
Despite diplomatic tensions rising and the Skripals and a police officer remaining in hospital, Russia tried to strike a humorous tone.
Its UK embassy tweeted a picture of fictional Belgian detective Hercule Poirot, writing: “In absence of evidence, we definitely need Poirot in Salisbury!”