The poisoning of Sergei Skripal and his daughter Yulia in the English city of Salisbury on March 4 has triggered a crisis with Britain and Russia expelling diplomats in tit-for-tat measures.
London and its allies have blamed Moscow for the attack which left the Skripals in critical condition after exposure to a nerve agent.
Technical experts from the Organisation for the Prohibition of Chemical Weapons will visit Britain on Monday to collect samples of the toxin, which London says is a Soviet-designed military grade nerve agent called Novichok.
"These will then be despatched to highly-reputable international laboratories selected by the OPCW for testing with results expected to take a minimum of two weeks," Britain's Foreign Office said in a statement.
President Vladimir Putin has rejected allegations Russia was behind the attack.
"It's complete drivel, rubbish, nonsense that somebody in Russia would allow themselves to do such a thing ahead of elections and the World Cup," Putin told supporters Sunday after winning a fourth term as president.
"We have destroyed all chemical weapons," he said, adding that Russia was ready to take part in the investigation.
Putin's comments came after British Foreign Secretary Boris Johnson accused Russia of stockpiling the nerve agent used in the attack.
"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," Johnson told the BBC's Andrew Marr Show.
'Sarcasm and denial'
Johnson said Britain would target wealth linked to the Kremlin in response to the poisoning of the former Russian double agent.
"Where people have obtained wealth by corruption and where we can see a link with the Kremlin, with Vladimir Putin, it may be possible to have unexplained wealth orders and other sanctions on those individuals," he said.
Johnson said the government was considering something similar to the US "Magnitsky Act" which was adopted in 2012 to punish Russian officials accused of human rights violations.
The act imposed a visa ban and froze the assets of Russian officials implicated in the death of lawyer Sergei Magnitsky, a tax fraud whistleblower who died in Russian custody in 2009.
Johnson accused the Russians of "smug sarcasm and denial" in response to the accusations, and said the international community was behind Britain.
Moscow's "malign, disruptive behaviour" internationally was the reason why allies were "inclined not to give Russia the benefit of the doubt," he added.
Johnson is expected to brief his EU counterparts on the case at a meeting of foreign ministers and NATO Secretary General Jens Stoltenberg in Brussels on Monday.
Russia's ambassador to the European Union, Vladimir Chizhov, has also insisted Moscow "had nothing to do" with the attack and appeared to suggest that Britain itself may have been the source of the chemical agent.
"When you have a nerve agent, you check it against certain samples you have in your laboratories," he said Saturday.
"And Porton Down, as we now all know, is the largest military facility in the UK that has been dealing with chemical weapons research and it's actually only eight miles from Salisbury."
Johnson called the accusation "satirical", adding it was "not the response of a country that really believes itself it to be innocent."
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