Corbyn had said Johnson had “serious questions to answer” and had “egg on his face” after the head of the Porton Down defence laboratory revealed on Tuesday that it had “not identified the precise source” of the nerve agent - a comment that appeared to contradict the Foreign Secretary’s previous statement.
The Foreign Office also deleted a tweet claiming Russia had produced the nerve agent.
In a series of tweets, the Foreign Secretary hit back.
He said: “It is lamentable that Jeremy Corbyn is now playing Russia’s game and trying to discredit the UK over Salisbury attack. Let’s remember the key facts:
“1) Porton Down identified nerve agent as military grade Novichok; 2) Russia has investigated delivering nerve agents, likely for assassination, & as part of this programme has produced and stockpiled small quantities of Novichoks; 3) Russia has motive for targeting Sergei Skripal.
“28 other countries have been so convinced by UK case they have expelled Russians. In contrast, Jeremy Corbyn chooses to side with the Russian spin machine.”
After the Sky News interview, critics seized on the contrast between the scientists’ caution and the Foreign Secretary’s interview with a German news channel in which he declared British defence experts had told him that the Kremlin was behind the attack on Sergei and Yulia Skripal.
Asked directly how he could assert that the source of the Novichok nerve agent was Russia, the Foreign Secretary replied that he had ‘interrogated’ the scientists.
He then told DW.com: “People from Porton Down, they were absolutely categorical. I asked the guy myself I said ‘are you sure?’ and he said there’s no doubt.”
Johnson appears to have been riled by Corbyn saying the Foreign Secretary has been “left with egg on his face”.
Corbyn told ITV News: “Boris Johnson seems to have completely exceeded the information that he had been given. And told the world in categorical terms what he believed had happened and it’s not backed up by the evidence he claimed to have got from Porton Down in the first place.
“Boris Johnson needs to answer some questions. The Foreign Secretary made a statement, the Foreign Office put out a tweet in support of what he said. Porton Down then said they couldn’t and wouldn’t identify where it had come from so they then deleted the tweet.
“Where does that leave the Foreign Secretary? Egg on his face for the statement he made on German television.”