Russia has been stockpiling the nerve agent used in the Salisbury spy attack for a decade, Boris Johnson has claimed.
Independent inspectors will arrive in the UK on Monday to test the substance used in the attack, but the results will take at least two weeks.
The Foreign Secretary said Moscow has investigated ways of delivering nerve agents for assassination over the last 10 years in a breach of international rules.
Investigators from the Organisation for the Prohibition of Chemical Weapons (OPCW) will visit the UK to take samples of Novichok, the substance used to attack former double agent Sergei Skripal, 66, and his daughter Yulia, 33.
The team from The Hague will use international laboratories to carry out tests on the nerve agent.
Mr Johnson will travel to Brussels to brief foreign ministers from across the European Union at a meeting on Monday on the attempted assassinations before holding talks Nato secretary general Jens Stoltenberg.
Labour has faced intense criticism for its response to the attack after leaving open the possibility that Russia was being framed.
Shadow chancellor John McDonnell said the Salisbury incident is “highly likely” to have been a state execution, and Russian president Vladimir Putin “is responsible” for the attack whether directly or through negligence.
He told ITV One’s Peston on Sunday: “He is responsible whichever way you look at it, he is responsible and all the evidence points to him.”
He added: “We support exactly what the Prime Minister said and we condemn Russia for this, condemn them. I believe this is a pattern of behaviour we have seen.”
Russia’s ambassador to the EU, Vladimir Chizhov, suggested the nerve agent may have come from the Porton Down laboratory, which is about eight miles from Salisbury.
Sweden and the Czech Republic denied Russian suggestions they may have been the source of the nerve agent.
Labour former prime minister Tony Blair said Theresa May has “done the right thing in relation” to the attack.
Speaking at the same event in Dubai, Tory former chancellor George Osborne, attacked Labour leader Jeremy Corbyn’s response.
“Jeremy Corbyn’s behaviour has frankly been disgraceful,” he said at the Global Education and Skills Forum.
“He has ended up sounding like an apologist for the Russian regime.”
The national security council will meet early next week to discuss Moscow’s tit-for-tat response to the UK’s expulsion of 23 Russian diplomats.
Russia’s ambassador to the EU, Vladimir Chizhov, prompted a strong rebuttal when he suggested the poison may have come from the Porton Down laboratory, which is around eight miles from Salisbury.
He told the BBC’s Andrew Marr Show that Russia had “nothing to do” with the incident, however his comments were rejected as “nonsense” by UK officials.
In an interview with the Mail on Sunday, Russian ambassador Alexander Yakovenko accused the Prime Minister of using the crisis to improve her image at the expense of relations with Moscow.
“In case of further unfriendly actions against Russia, the Russian side reserves the right to take further retaliatory measures – this is what the British Ambassador was told on Saturday,” he said.