Salisbury attack: Boris Johnson refuses to back down over claim Porton Down had found novichok came from Russia

Accused of misleading the public, Foreign Secretary instead lashes out at Jeremy Corbyn for 'playing Russia’s game and trying to discredit the UK'

Boris Johnson has refused to back down after wrongly claiming Porton Down experts had identified Russia as the source of the Salisbury nerve agent – instead accusing Jeremy Corbyn of siding with Moscow.

The Foreign Secretary came out fighting after Labour said he had “misled the public”, turning on its leader for “playing Russia’s game and trying to discredit the UK”, in a series of provocative tweets.

The Labour leader had warned Mr Johnson had “some very serious questions to answer”, following the defence laboratory’s admission that it could not identify the origin of the novichok used.

But, in his tweets, Mr Johnson made no mention of his controversial interview with a German TV station, in which he had claimed Porton Down had been “absolutely categorical” in its verdict.

Instead, he launched a fierce attack, saying: “It is lamentable that Jeremy Corbyn is now playing Russia’s game and trying to discredit the UK over Salisbury attack.”

The Foreign Secretary insisted Russia had “produced and stockpiled small quantities of Novichoks” and argued it had a “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,” Mr Johnson added.

The controversy blew up after the broadcaster Deutsche Welle, two weeks ago, asked Mr Johnson 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.”

In fact, as the Porton Down chief executive acknowledged on Tuesday, the centre had been unable to determine if the novichok had been produced in Russia.

Diane Abbott, Labour's shadow home secretary, said the Foreign Secretary had “misled the public”, while Mr Corbyn said: “Boris Johnson has some very serious questions to answer.”

The Foreign Office was also embarrassed after admitting it had deleted an “inaccurate” tweet stating that scientists had concluded the novichok used in the attack had been “produced in Russia”.

It 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.”

The Foreign Office blamed mistakes in tweeting a briefing given by the UK’s Ambassador to Russia “in real time” for its blunder.

“One of the tweets was truncated and did not accurately report our Ambassador’s words. We have removed this tweet,” a spokeswoman said.

To add to the embarrassment, the deleted tweet was spotted by the Russian Embassy in London – handing it a publicity coup and further ammunition for its claim that the UK has been dishonest.

Meanwhile, the security minister Ben Wallace went further than previous comments by ministers – or by Porton Down – insisting Russia was the only producer of novichok.

Mr Wallace rejected claims that the contradictory statements about Salisbury amounted to a “mess”, telling the BBC: “I don’t think so at all.”

And he added: “Who did design novichok? That was the Russians, who have developed and stockpiled it. In fact, the cast of that is reduced to one.”