"It is our job to provide the scientific evidence of what this particular nerve agent is, we identified that it is from this particular family and that it is a military grade, but it is not our job to say where it was manufactured", Aitkenhead said.
The defence research facility, which identified the substance in Salisbury as Novichok, said it was likely to have been deployed by a "state actor".
He added: "we are continuing to work to help to provide additional information that might help us get closer to [the source] but we haven't yet been able to do that".
"We hope that during those discussions a full stop will be placed on [the issue of] what happened" to former spy Sergei Skripal, Mr Putin said in Ankara ahead of the meeting on Wednesday of the Organisation for the Prohibition of Chemical Weapons (OPCW).
"This could be in the interests of the British government which found itself in an uncomfortable situation having failed to fulfil promises to its electorate about the conditions for Brexit", Lavrov said at a press conference in Moscow, referring to Britain's planned departure from the European Union.
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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 Russian Federation was based on "a number of other sources". Yulia is no longer in critical condition.
Prior to Friday's expulsions, the Kremlin announced that 60 U.S. diplomats would be kicked out of the country, and the USA consulate in St. Petersburg shut in retaliation for a similar step by Washington, which said Monday it would be expelling the same number of Russian diplomats and closing the Russian consulate in Seattle.
Russian Federation requested the meeting and has demanded an "unbiased investigation" by the Organization for the Prohibition of Chemical Weapons (OPCW).
In comments reported by Russian news agencies, Mr Grushko said the attempted murders could have been "arranged by Britain" because "they need a major enemy".
If tensions between Russia and its Western counterparts continue, they could lead to "the last war in the history of mankind", a former Russian general has warned.
On March 4, former Russian military intelligence (GRU) Colonel Sergei Skripal, who had been convicted in Russia of spying for Great Britain and later swapped for Russian intelligence officers, and his daughter Yulia suffered the effects of an alleged nerve agent in the British city of Salisbury.
Russia's embassy in the United Kingdom said it "understood from the very start" that Whitehall statements on the nerve agent having been produced in Russian Federation were "a bluff".
"You say Russian Federation should change its behaviour, it's not the kind of talk or compromise we need".