BORIS Johnson has become embroiled in a war of words with the head of Britain’s statistics watchdog over the Foreign Secretary’s remarks about a £350 million a week Brexit dividend.

The row flared up after Amber Rudd, the Home Secretary, had earlier ticked off her Cabinet colleague for “back-seat driving” on Brexit following his 4,000-word article at the weekend, which has angered senior Tories as it comes just days before Theresa May is due to give a keynote speech on Britain’s EU withdrawal in Florence.

Mr Johnson’s spat with Sir David Norgrove began when the Chairman of the UK Statistics Authority wrote to the Secretary of State, saying how he was "surprised and disappointed" that Mr Johnson had allegedly reused the widely-discredited referendum pledge that up to £350m a week extra could be spent on the NHS after Brexit, stressing that it was a "clear misuse" of official figures.

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But a spokesman for the Foreign Secretary then intervened to say: "Boris has spoken to Norgrove and he has made clear that he was complaining about the headlines and not Boris' piece and, in fact, admitted that Boris' wording in the piece was absolutely fine."

However, later, a spokesman for the statistics authority responded by stressing: "Sir David Norgrove does not believe the issues lie solely with the headlines. He has not changed the conclusion set out in his letter to the Foreign Secretary."

In an unusual move, Mr Johnson then personally and publicly hit back, accusing Sir David of a "complete misrepresentation" of his claims about Brexit and called on him to withdraw his criticism.

In an angry letter to the statistics authority chairman, the Secretary of State wrote: “I must say that I was surprised and disappointed by your letter of today, since it was based on what appeared to be a wilful distortion of the text of my article.

"When we spoke you conceded that you were more concerned by the headline and the BBC coverage, though you accepted that I was not responsible for those. I suggest if the BBC coverage offends you that you write to the BBC.

"You say that I claim that there would be £350m that 'might be available for extra public spending' when we leave the EU.

"This is a complete misrepresentation of what I said and I would like you to withdraw it. I in fact said: 'Once we have settled our accounts we will take back control of roughly £350m per week. It would be a fine thing, as many of us have pointed out, if a lot of that money went on the NHS.'

"That is very different from claiming that there would be an extra £350m available for public spending and I am amazed that you should impute such a statement to me," added Mr Johnson.

Meanwhile, Labour's Chuka Umunna, a supporter of Open Britain group campaigning for a soft Brexit, said: "Yet again, Boris's outright lying has been exposed by Britain's statistics watchdog. The £350m figure was simply wrong during the referendum campaign and it's wrong now.

"Boris' hard Brexit plans will mean less money for our NHS, not more. The IFS are forecasting a £58bn hole in the public finances as a direct result of Brexit, which will be paid for in higher taxes and lower NHS spending.

"Boris's compulsive lying means he has lost the right to be heard on Brexit. He should give his leadership ambitions a rest and apologise for his continual use of dodgy statistics."

Sir Vince Cable, the Liberal Democrat leader, said: "Boris' £350m lie has been exposed yet again. He knows an extreme Brexit would damage the economy and mean less cash for the NHS.

"I'm glad to see the independent UK Statistics Authority has the courage to slap Boris down. It's a shame the same can't be said of Theresa May," he added.