The foreign secretary came under pressure as it emerged the Foreign Office also deleted a tweet stating that scientists had concluded the novichok used in the attack had been “produced in Russia”.
In fact, as the Porton Down chief executive acknowledged on Tuesday, the defence laboratory had been unable to pinpoint the “precise source” of the deadly agent.
The tweet – which the Foreign Office admitted did not “accurately” reflect the information it had received – was published on 22 March.
Around the same date, Mr Johnson gave an interview to Deutsche Welle, the German broadcaster, in which he was asked how the UK had been able to determine so quickly that the novichok came from Russia.
He replied: “When I look at the evidence, the people from Porton Down, the laboratory, they were absolutely categorical.
“I asked the guy myself. I said: ‘Are you sure?’ And he said: ‘There’s no doubt.’ So we have very little alternative but to take the action that we have taken.”
Diane Abbott, Labour’s shadow home secretary, said: “It seems Boris Johnson misled the public when he claimed that Porton Down officials confirmed to him that Russia was the source of the nerve agent used in the Salisbury attack.
“Those officials have made it clear they cannot identify its source, and are not able to definitively say it came from Russia or elsewhere.
“Boris Johnson is supposed to represent Britain on the world stage, but time and again he has shown he is unable to do so responsibly.”
Meanwhile, an embarrassed Foreign Office blamed the mistake on it tweeting a briefing given by the UK’s Ambassador to Russia “in real time” for the blunder.
The now-deleted tweet read: “Analysis by world-leading experts at the Defence Science and Technology Laboratory at Porton Down made clear this was a military-grade Novichok nerve agent produced in Russia.”
“One of the tweets was truncated and did not accurately report our ambassador’s words. We have removed this tweet,” a spokeswoman said.
She added that the Foreign Office had been attempting to reach “as wide an audience as possible” by tweeting excerpts from the briefing as quickly as possible.
Gary Aitkenhead, the head of the Defence Science and Technology Laboratory, caused dismay in No 10 when he said he had not identified the “precise source” of the novichok used.
Porton Down had identified it as a military-grade nerve agent, which could probably be deployed only by a nation state, but could not prove it came from Russia, he said.
Downing Street was forced into a damage limitation exercise, arguing the Porton Down tests were “only one part of the intelligence picture” and insisting there was still “no other plausible explanation” than Russian culpability.
Jeremy Corbyn added to the criticism of the foreign secretary, telling Sky News: “Boris Johnson has some very serious questions to answer.
“He claimed on German television that this was a Russian-produced nerve agent and Porton Down then examined it and said all they could identify it was as novichok.
“They couldn’t say where it came from, and the chemical weapons organisation is meeting to continue that discussion about where it goes from here.”
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