- Video report by ITV News correspondent Romilly Weeks
The UK has evidence Russia had been developing and stockpiling the nerve agent Novichok over the past decade, Foreign Secretary Boris Johnson has said.
Independent inspectors will arrive in the UK on Monday to test the substance used in the Salisbury spy attack, though the results will take at least two weeks.
The foreign secretary said Moscow had breached international rules by investigating ways of delivering nerve agents for assassination.
And he dismissed a Russian claim the Novichok poison may have come from a British laboratory around eight miles from Salisbury.
Shadow chancellor John McDonnell meanwhile told ITV's Peston on Sunday Labour believed the Salisbury attack is "highly likely" to have been a state execution, adding: "whichever way you look at it he [Vladimir Putin] is responsible".
Who claimed Novichok could have been made in the UK?
Russia's ambassador to the EU suggested the Novichok nerve agent may have come from the Porton Down laboratory, which is around eight miles from Salisbury.
Vladimir Chizhov told the BBC's Andrew Marr Show that Russia had "nothing to do" with the incident and had no stockpiles of chemical weapons left.
He went on: "Porton Down, as we now all know, is the largest military facility in the United Kingdom that has been dealing with chemical weapons research. It's actually only eight miles from Salisbury."
Asked whether Porton Down was "responsible" for the attack, Mr Chizhov said: "I don't know. I don't have any evidence of anything having been used."
Salisbury MP John Glen branded Mr Chizhov's suggestions "outrageous" and "laughable if it wasn’t for the dire consequences of his regime’s nerve agents".
When he appeared on Marr, Mr Johnson branded Mr Chizhov's statement "smug" and "not the response of a country that believes itself to be innocent".
What did Boris Johnson say on the stockpiling?
Mr Johnson said: "We actually have evidence within the last ten 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."
He said experts from the Organisation for the Prohibition of Chemical Weapons will come to the UK on Monday to begin testing samples of the nerve agent used on the former Russian spy, Sergei Skripal, and his daughter, Yulia.
The Foreign Office later confirmed the tests will take a "minimum of two weeks".
What else has the foreign secretary said?
Mr Johnson said he was in the "Kremlin's crosshairs" because Britain is the country that has "time and again called Russia out".
The foreign secretary said the reaction of countries such as the US, Germany, France and the Baltic countries in the wake of Mr Skripal's poisoning was vastly different to their "more hesitant" reaction in the wake of the death of Alexander Litvinenko, because they had all experienced "Russian meddling, malign, disruptive, Russian behaviour.
"They can see a country that is going in the wrong direction and that's why they are so inclined now not to give Russia the benefit of the doubt and to stand shoulder to shoulder with the UK."
The 53-year-old continued that Britain would be pursuing Russians who had corruptly obtained their wealth, but that this was a matter for law enforcement authorities, it would not be up to the Government to "direct authorities to go after individuals" as "that's what they do in Russia".
But he added that that in the wake of the attack, ordinary Russians should not be demonised.
He separately branded Russia's counter-measures "futile", after the Kremlin expelled 23 British diplomats from the country, as the row over the Salisbury attack intensified.
Writing in the Sun on Sunday, Mr Johnson said the Kremlin's tit-for-tat expulsion of diplomats and the closures of the British consulate in St Petersburg and the British Council would only impact Vladimir Putin's compatriots, adding that while the UK has "friends across the world", Russia "does not".
What has been the Russian reaction?
Russia's ambassador to the UK warned the dispute was escalating "dangerously and out of proportion" and the country reserved the right to take "further retaliatory measures" if more sanctions are implemented.
In a message put out after the interviews, the Russian embassy in London tweeted a picture of Agatha Christie's fictional Detective Poirot, saying he was needed in Salisbury "in absence" of any "evidence".
The row comes as Russia heads to the polls in the country's presidential election.