Gary Aitkenhead Chief Executive of the Porton Down defence laboratory handling the case investigation said that analysts had identified it (nerve agent) as military-grade Novichok but they had not proved that it was made in Russian Federation as per news outlet France 24.
"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 you have come to".
He said establishing its origin required "other inputs", some of them intelligence-based, that the Government has access to.
Britain's defense laboratory acknowledged Tuesday it hasn't tracked down the source of the nerve agent that poisoned a Russian ex-spy, a statement the Kremlin said proved that British accusations of Moscow's involvement were baseless.
A United Kingdom government spokesperson said in a statement Tuesday: "We have been clear from the very beginning that our world-leading experts at Porton Down identified the substance used in Salisbury as a Novichok, a military grade nerve agent".
He added: "It is our job to provide the scientific evidence of what this particular nerve agent is".
He said: "It is a cold war".
Aitkenhead said there was "no way" the nerve agent could have come from the high-security facility.
Russian officials also have suggested the poison could have come from Britain, pointing out that Porton Down conducts secret chemical and biological weapons research.
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Sergei Skripal, a former Russian intelligence agent convicted of spying for Britain, and his daughter were found unconscious and critically ill in the English cathedral city of Salisbury a month ago.
Filatov says Russian Federation wants Britain to "provide every possible element of evidence" it holds about the March 4 attack on Sergei Skripal and his daughter Yulia in the English city of Salisbury.
US President Donald Trump defended his embrace of Vladimir Putin Tuesday, saying good relations were important but claiming no one had been tougher on Russian Federation.
On Tuesday, the Organization for the Prohibition of Chemical Weapons said it would hold an emergency meeting on the case at the request of Russian Federation, which is demanding Britain hand over its evidence.
Later, London claimed that the toxin of Novichok-class had been allegedly developed in Russian Federation. British authorities suspect Skripal was poisoned by a Soviet-made nerve agent.
Of Yulia's current condition, she said: 'The only thing I discovered from our (Russian) diplomats is that Yulia has opened her eyes, can eat, drink and has even said a few words'.
Evgeny Buzhinskiy - who served in the Russian military for more than 40 years - said the row between Britain and Putin was creating a situation "worse than the Cold War".
He said that if Britain does not show evidence to back up its allegation, "there are ample grounds to assume that we are dealing with a grand scale provocation organized in London aimed to discredit Russian Federation".
On what the issue meant for the relationship between Ireland and Russia, Mr Filatov said he did not think it was really "a huge story".