Russia will take its complaints over the Salisbury nerve agent attack to The Hague today after the Porton Down military research facility said it could not trace the agent's precise source.
An extraordinary meeting of the Organisation for the Prohibition of Chemical Weapons (OPCW) has been called by Russia to "address the situation around allegations of non-compliance" with the chemical weapons convention made by the UK against Moscow.
Russia has denied responsibility and demanded access to the British investigation into the March 4 attack on former spy Sergei Skripal and his daughter Yulia.
The Foreign Office accused Russia of attempting to undermine the work of the OPCW and engaging in "another diversionary tactic".
It comes a day after the head of Porton Down said his scientists have not verified that the nerve agent used in Salisbury came from Russia.
A Foreign Office spokesman said: "Russia has called this meeting to undermine the work of the OPCW, which, fully in accordance with the chemical weapons convention, is providing the UK with technical assistance and evaluation through independent analysis of samples from the Salisbury attack.
"Of course, there is no requirement in the chemical weapons convention for the victim of a chemical weapons attack to engage in a joint investigation with the likely perpetrator.
"This Russian initiative is yet again another diversionary tactic, intended to undermine the work of the OPCW in reaching a conclusion."
Wednesday's meeting of the OPCW executive council at The Hague will be held behind closed doors.
Gary Aitkenhead, the chief executive of the Government's Defence Science and Technology Laboratory (DSTL), said the poison used in Salisbury had been identified as a military-grade Novichok nerve agent which could probably be deployed only by a nation-state.
But he told Sky News it was not Porton Down's role to work out where the agent came from and suggested the Government's conclusion that it was highly likely to have come from Russia was based on "a number of other sources".
President Vladimir Putin, citing Mr Aitkenhead, called for a thorough investigation into the poisoning during a visit to Turkey, where he said "the speed at which the anti-Russian campaign has been launched causes bewilderment."
Porton Down's identification of the substance used in the attack on the Skripals as Novichok was a key plank in the evidence presented by the UK in Theresa May's successful bid to recruit international support in the dispute with Moscow, resulting in the expulsion of more than 100 Russian diplomats from over 20 countries.
German politician Armin Laschet, an ally of Chancellor Angela Merkel, said: "If one forces nearly all Nato countries into solidarity, shouldn't one have certain evidence?
"Regardless of what one thinks about Russia, my study of international law taught me a different way to deal with other states."
Mr Aitkenhead also rejected Russian claims the substance could have come from Porton Down.
A Downing Street spokesman said: "As the Prime Minister has made clear, the UK would much rather have in Russia a constructive partner ready to play by the rules.
"But this attack in Salisbury was part of a pattern of increasingly aggressive Russian behaviour, as well as a new and dangerous phase in Russian activity within the continent and beyond."