"Egg on his face for the statement he made on German television".
The Labour leader said that Mr Johnson has "serious questions" to answer after saying in an interview a fortnight ago that he had been told by Porton Down scientists that the nerve agent used in the poisoning was made in Russian Federation.
Mr Johnson was asked if they had samples.
Among those hauling Johnson over the coals for his DW interview was the oppostion Labour Party Shadow Home Secretary Diane Abbott, who retweeted a post featuring an edited clip from the interview.
The Foreign Office told the HuffPost it had deleted the tweet, which was recounting a Moscow briefing by British Ambassador Laurie Bristow, because it "was truncated and did not accurately report our Ambassador's words".
Britain's Porton Down biological and chemical weapons laboratory on Tuesday announced that the toxin was in a category of Soviet-era nerve agents called Novichok, although they could not yet determine whether it was made in Russian Federation.
"This is only one part of the intelligence picture", the statement from Prime Minister Theresa May's office said, pointing toward "our knowledge that within the last decade, Russian Federation has investigated ways of delivering nerve agents probably for assassination - and as part of this program has produced and stockpiled small quantities of Novichoks", among other factors. "We have removed this tweet", he explained. "And they were absolutely categorical and I asked the guy myself, I said, "Are you sure?' And he said, 'There's no doubt".
But during a local election campaign visit to Watford, the Labour leader accused Mr Johnson of exaggerating the evidence. Porton Down have not said that.
He said: "When I look at the evidence, I mean the people from Porton Down, the laboratory". They can not identify the source of it.
"Where does that leave the Foreign Secretary?" I don't know which it is, but I think we need a responsible, cool approach to this.
LabourList has more readers than ever before - but we need your support. Boris Johnson needs to answer some questions.
But Jeremy Corbyn suggested the Foreign Secretary was left with "egg on his face" after the defence research laboratory admitted it could not identify the source of the poison.
"1) Porton Down identified nerve agent as military grade Novichok; 2) Russian Federation has investigated delivering nerve agents, likely for assassination, & as part of this programme has produced and stockpiled small quantities of Novichoks; 3) Russian Federation has motive for targeting Sergei Skripal".
Boris Johnson has "serious questions to answer" over his statements claiming the Novichok nerve agent used in the Salisbury attack came from Russian Federation, says Jeremy Corbyn.
Meanwhile, after Russia President Vladimir Putin accused the United Kingdom of mounting an "anti-Russian campaign", Sergei Naryshkin, Director of Russia's foreign intelligence service, described Britain's claims that Moscow was to blame for the Salisbury poisoning as a "grotesque provocation rudely staged by the British and USA intelligence agencies".
Sky News tweeted #Salisburyattack: Scientists have not been able to prove that Russian Federation made the nerve agent used in the spy poisoning.