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Boris Johnson has launched a vicious new attack on Jeremy Corbyn, branding him "infantile" and "the Kremlin's useful idiot".

The Foreign Secretary accused Labour's leader of joining Russia's "propaganda onslaught" by failing to challenge their "avalanche of lies" about the Salisbury spy attack.

Labour hit back by accusing Mr Johnson of turning to "ridiculous insults" to distract from the fact he misled the public, made a fool of himself and undermined the government.

Mr Johnson was red-faced this week when the head of the Porton Down chemical weapons lab said his team had not traced the Novichoknerve agent , used in Salisbury, to Russia.

The government insisted that was never Porton Down's responsibility - and it drew the conclusion from multiple intelligence sources.

Yet Mr Johnson had previously implied Porton Down traced the nerve agent used on ex-spy Sergei Skripal to Russia.

And the Foreign Office deleted a tweet making that same claim.

Labour seized on the conflicting messages - but so did Russia.

(Image: Getty) (Image: Getty Images Europe)

The Kremlin tried to open investigations at the United Nations and the world chemical weapons watchdog in The Hague.

And Russia's ambassador to the UK held a bizarre 90-minute press conference declaring: "We hear all the stories and the theories about our motivations. We don't buy it."

Writing in the Sunday Times today, Mr Johnson took aim at Mr Corbyn, who has refused to definitively blame Russia saying more evidence is needed.

Tory minister Ben Wallace this week admitted some intelligence about the attack may have been withheld from Mr Corbyn, despite his membership of the high-level Privy Council.

Yet the Foreign Secretary said that by questioning the evidence, the Labour leader was playing "Putin's game" and "shames himself by lending it succour".

He blasted Mr Corbyn's "infantile leftist background of sympathy" for any country hostile to Britain.

(Image: REUTERS)

Mr Johnson wrote: "There is only one thing that gives the Kremlin succour and lends false credibility to its propaganda onslaught.

"That is when politicians from the targeted countries join in the effort - whether wittingly or not - to create doubt where there is none and sow confusion where there is clarity..

"Sadly, I am driven to the conclusion that Jeremy Corbyn has joined this effort."

Attacking Russia's claims, Mr Johnson added: "I will not mince words.

(Image: Getty)

"This is cynical nonsense and Corbyn shames himself by lending it succour.

"Truly he is the Kremlin's useful idiot."

A Labour spokesman responded: "Jeremy Corbyn has repeatedly said the evidence points to Russia being responsible, directly or indirectly, and that the Russian authorities must be held to account on the basis of evidence.

"Boris Johnson has made a fool of himself and undermined the Government by seriously misrepresenting what he was told by Porton Down chemical weapons experts.

"These ridiculous insults won't distract attention from the fact that he has clearly misled the public over vital issues of national security."

Today former Deputy Prime Minister John Prescott sides with Mr Corbyn, accusing Labour MPs of "backing the Government’s line on the Salisbury attacks without conclusive evidence".

Meanwhile today the Russian Embassy has accused the Foreign Office (FCO) of failing to answer its questions over why a relative of Sergei and Yulia Skripal was denied a visa to visit them as their condition improves a month after the attack left them in comas.

In a statement, the embassy said the FCO's response to its request for an explanation was "disappointing" and "cannot but cause regret".

Russian officials said Viktoria Skripal, a cousin of Ms Skripal, applied for a visa at the British embassy in Moscow.

UK authorities refused to grant her a visa, with the Home Office saying her application "did not comply with the immigration rules", prompting a suggestion from the Russian side that the British had "something to hide".

On Sunday the Russian embassy repeated its charge that the decision not to issue a visa was "politically motivated" and raised questions about the reasoning behind it.

(Image: AFP)

Ms Skripal told Sky News she wanted to visit the pair in person so she could "truthfully" report back on their condition to relatives.

"But my visa was rejected," she said.

"The whole world is now talking about an unprecedented political scandal, but real people are at the epicentre of this scandal. This is our family, which really needs to be together now.

"If Yulia sees this, I want her to see we love her and will always wait for her. I will fight for her till the end.

"If I can't do it here, I will go to the United Nations, and if I can't do it this way, I will walk, I will hitchhike across the border illegally."