"We have not verified the precise source", Gary Aitkenhead of the Defence Science and Technology Laboratory told British broadcaster Sky News on Tuesday.
"We have not identified the precise source, but we have provided the scientific info to government who have then used a number of other sources to piece together the conclusions", he said.
London has directly accused the Kremlin on at least three occasions of being behind the chemical attack on former double agent Sergei Skripal and his daughter, Yulia.
Speaking to German television, Mr. Corbyn said the foreign secretary stated he had categorical assurances linking the nerve agent to Russian Federation by Porton Down.
"I would like to inform you, although we have already told [you] about that, we are convening a OPCW Executive Council session tomorrow to look into the situation in detail", Putin said during a joint press conference with Turkish President Recep Tayyip Erdogan in Ankara.
On this basis, Britain sought and won the expulsion of 130 Russian diplomats from 26 countries-most notably the US-and demanded the acceleration of North Atlantic Treaty Organisation efforts against Russia's "destabilising activities". He added: 'The speed at which the anti-Russian campaign has been launched causes bewilderment'.
But Britain's foreign ministry accused Russian Federation of requesting the meeting to undermine the organisation's investigation.
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"It is our job to provide the scientific evidence of what this particular nerve agent is".
He only went so far as to note that the sample they studied is "probably only within the capabilities of a state actor".
As the fallout from the incident continues, both Skripal and his daughter remain hospitalized in Salisbury.
Most of these people also transformed into armchair generals as soon as the Iraq war was mooted, an act that predictably became Britain's biggest foreign policy disaster of the last 30 years.
Asked by Sky News to respond to such a grave allegation, Aitkenhead's comments were especially noteworthy.
On Tuesday, the Organization for the Prohibition of Chemical Weapons said it would hold an emergency meeting on the case Wednesday at its headquarters in The Hague at the request of Russian Federation, which is demanding Britain hand over its evidence.
On March 20 - the same day the United Kingdom formally expelled 23 Russian diplomats for the Kremlin's alleged role in the Skripals' poisoning - Johnson told DW that researchers at the laboratory "were absolutely categorical" about the evidence.
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Russian officials have claimed the poison may have come from Britain, pointing out that Porton Down conducts secret chemical and biological weapons research.
"Either the Foreign Secretary has information that he's not sharing with Porton Down, or it was a bit of exaggeration", he said.
Mr Aitkenhead also denied Russian claims the agent may have been made at Porton Down itself.
"Unfortunately this is an extremely toxic substance".
'There is no way anything like that could have come from us or left the four walls of our facility, ' he said.
But Russia's president Vladimir Putin said last night "20 countries" could make novichok.
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