"Porton Down have not said that, they said that they've identified it as Novichok, they can not identify the source of it".
Mr Johnson had been under pressure since the head of the Porton Down chemical weapons research centre said it could not verify the precise source of the Novichok nerve agent used in the attack.
However, Aitkenhead said that the nerve agent would require "extremely sophisticated methods to create, something only in the capabilities of a state actor".
Speaking during a local election campaign visit to Watford, Mr Corbyn said: "He claimed categorically - and I think he used the words 101% - that it had come from Russian Federation".
Williamson has called on MPs to apologize to Labour leader Jeremy Corbyn, who demanded that all the facts be made clear before laying blame on Russian Federation or any party. "I think Jeremy Corbyn was absolutely right to urge caution and to ask for clear evidence before we start to raise global tensions", Williamson said.
Labour, which is the UK's main opposition party, pointed to an interview given to German TV in which Mr Johnson was challenged on his claim that Russian Federation was the source of the Novichok nerve agent.
The tweet, issued on March 22, said: 'Analysis by world-leading experts at the Defence Science and Technology Laboratory at Porton Down made clear that this was a military-grade Novichok nerve agent produced in Russian Federation'. "With egg on his face".
In an awkward development for the Foreign Office, the Russian embassy's Twitter feed pointed out that the 20 March tweet on a presentation by Britain's ambassador to Moscow on the Salisbury attack had disappeared.
In a series of tweets, Mr Johnson said: 'It is lamentable that Jeremy Corbyn is now playing Russia's game and trying to discredit the United Kingdom over Salisbury attack. In the aftermath of the attack, Moscow had offered a dizzying array of alternative theories for the poisoning, prompting Johnson to say on March 19 Russian Federation was trying to "conceal the needle of truth in a haystack of lies and obfuscation".
Mr Johnson listed his evidence for blaming the Kremlin, including its production of Novichok and motive for targetting Sergei Skripal. So, Johnson "needs to answer some questions", Corbyn emphasized, because "there were clearly huge inconsistencies" in his story.
However, it emerged on Wednesday that the Foreign Office had earlier deleted a tweet claiming the British scientists had concluded that the nerve agent was "produced in Russia".
He added: "28 other countries have been so convinced by United Kingdom case they have expelled Russians". Moscow has responded with its own expulsions.