Update 7.40pm: The head of the UK statistics watchdog has refused to back down in the public spat with the British Foreign Secretary over his Brexit claims.
Boris Johnson accused David Norgrove of a "complete misrepresentation" of his views and called on him to withdraw the criticism.
But the UK Statistics Authority chairman "stands by" his accusation that Mr Johnson's decision to revive the controversial claim that up to £350m a week extra would be freed up for public spending after Brexit was a "clear misuse" of official figures.
A spokesman said the Foreign Secretary's response "doesn't alter his view", adding: "Sir David stands by his letter."
The row erupted after Norgrove published a letter to Mr Johnson saying he was "surprised and disappointed" the £350m figure had been revisited in his 4,000-word Brexit blueprint.
BREAKING: UK Stats Authority writes to Boris saying it's "surprised & disappointed" he's using £350m figure AGAIN. "Clear misuse of stats" pic.twitter.com/sen2BV4oFr— Paul Brand (@PaulBrandITV) September 17, 2017
He stood by the criticism when the Foreign Secretary's aides later claimed he was "absolutely fine" with the piece.
Mr Johnson then penned his own letter, which claimed the statistics boss had privately conceded he was "more concerned" by the headlines in the coverage" of the controversial article and "accepted that I was not responsible for those".
He 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 £350 million 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."
Boris Johnson's future in the British Cabinet has come under question after he set out a detailed vision of Britain's future outside the European Union that has been viewed as a challenge to Theresa May's leadership.
The British Foreign Secretary publicly declared he is "all behind Theresa for a glorious Brexit" after the 4,000-word assessment of Brexit fuelled speculation about his ambitions.
Mr Johnson revived the widely-criticised claim quitting the EU could boost NHS coffers by up to £350 million a week and warned against paying for access to European markets in the future.
But the in-depth article on life after March 2019 was released just six days before the Prime Minister sets out her Brexit blueprint in a speech in Florence.
It has prompted claims in the Mail on Sunday that allies of the PM believed the move was "hostile" and "attention seeking".
A senior minister who backed Remain told the Sunday Telegraph, meanwhile, Mr Johnson "needs to go and do something else" if he "can't settle" into his role as foreign secretary while a former minister said he was "sailing within an inch of being thrown out of the Government".
Craig Oliver, former director of communications at Downing Street for David Cameron, said even if the genuine intention was to support the British Prime Minister, it was obvious it would be seen in Westminster as a "direct challenge".
But as the furore raged, Mr Johnson insisted in a tweet he was "looking forward to PM's Florence speech".
"All behind Theresa for a glorious Brexit," he added.
Looking forward to PM's Florence speech. All behind Theresa for a glorious Brexit: https://t.co/5pe1pY2m13— Boris Johnson (@BorisJohnson) September 16, 2017
In his plan for Brexit, Mr Johnson suggested continued membership of the single market and customs union would make a "complete mockery" of the referendum result.
Writing for the Daily Telegraph, he said the Government should "seize the opportunity" of Brexit to reform the UK tax system to boost investment, he suggested.
Rules restricting foreign buyers from snapping up property could be imposed once Britain leaves the EU but the UK should not slam the door on immigration, he wrote.