The Bank of England governor has rubbished a key Boris Johnson claim for avoiding economic damage after a no-deal Brexit, in a severe blow to the Tory leadership favourite.

Mr Johnson dismissed fears about crashing out of the EU by arguing the UK could still enjoy tariff-free trade under what is known as “Gatt 24” until a permanent agreement was struck.

But Mark Carney pointed out the trade law could only be invoked if there was an agreement in place – and the central point of a no-deal Brexit was the absence of a deal.

“The Gatt rules are clear,” he told the BBC. “Gatt 24 applies if you have an [withdrawal] agreement, not if you’ve decided not to have an agreement, or you have been unable to come to an agreement.

“We should be clear that not having an agreement with the European Union would mean that there are tariffs, automatically – because the Europeans have to apply the same rules to us as they apply to everyone else.”

Alistair Burt, a former minister and supporter of Jeremy Hunt, Mr Johnson’s rival, said: “You have got to be on top of the detail – no deal would be very dangerous.”

Biggest lies told by Boris Johnson Show all 5 1 /5 Biggest lies told by Boris Johnson Biggest lies told by Boris Johnson Made-up quote for The Times Johnson was sacked from The Times newspaper in the late 1980s after he fabricated a quote from his godfather, the historian Colin Lucas, for a front-page article about the discovery of Edward II’s Rose Palace. “The trouble was that somewhere in my copy I managed to attribute to Colin the view that Edward II and Piers Gaveston would have been cavorting together in the Rose Palace,” he claimed. Alas, Gaveston was executed 13 years before the palace was built. “It was very nasty,” Mr Johnson added, before attempting to downplay it as nothing more than a schoolboy blunder. PA Biggest lies told by Boris Johnson Sacked from cabinet over cheating lie Michael Howard gave Boris Johnson two new jobs after becoming leader of the Conservatives in 2003 – party vice-chairman and shadow arts minister. He was sacked from both positions in November 2004 after assuring Mr Howard that tabloid reports of his affair with Spectator columnist Petronella Wyatt were false and an “inverted pyramid of piffle”. When the story was found to be true, he refused to resign. PA Biggest lies told by Boris Johnson Broken promise to boss In 1999 Johnson was offered editorship of The Spectator by owner Conrad Black on the condition that he would not stand as an MP while in the post. In 2001 he stood - and was elected - MP for Henley, though Black did allow him to continue as editor despite calling "ineffably duplicitous" PA Biggest lies told by Boris Johnson Misrepresenting the people of Liverpool As editor of The Spectator, he was forced to apologise for an article in the magazine which blamed drunken Liverpool fans for the 1989 Hillsborough disaster and suggested that the people of the city were wallowing in their victim status. “Anyone, journalist or politician, should say sorry to the people of Liverpool – as I do – for misrepresenting what happened at Hillsborough,” he said. PA Biggest lies told by Boris Johnson ‘I didn’t say anything about Turkey’ Johnson claimed in January, that he did not mention Turkey during the EU referendum campaign. In fact, he co-signed a letter stating that “the only way to avoid having common borders with Turkey is to vote Leave and take back control”. The Vote Leave campaign also produced a poster reading: “Turkey (population 76 million) is joining the EU”

He pointed out the gaffe came after Mr Johnson had stumbled over his plan for huge tax cuts for high earners when the impact on taxpayers in Scotland was revealed.

To use Article 24 of the General Agreement on Tariffs and Trade (Gatt) – avoiding tariffs on goods – a trade agreement must be agreed in principle, rather than be an aspiration.

Its use also needs the two sides to agree, meaning the UK could not simply impose it on the EU after a crash-out departure.

During Tuesday night’s TV debate, Mr Johnson was challenged by Rory Stewart on the import taxes – and, therefore, border controls – that would be required on agricultural goods crossing to and from the Republic of Ireland.

He replied: “There will be no tariffs, there will be no quotas, because what we want to do is get a standstill in our current arrangements under Gatt 24 – or whatever it happens to be – until such time that we have negotiated an FTA [Free Trade Agreement].”

The answer appeared to betray the former foreign secretary’s lack of detailed knowledge, a persistent criticism of his record in office.

The government has already accepted there would be some tariffs, after a no-deal Brexit, on beef, lamb, pork, poultry and some dairy products, finished vehicles and ceramics.

Carolyn Fairbairn, the head of the Confederation of British Industry employers’ organisation, described the March announcement as “a sledgehammer for our economy”.

Furthermore, even the absence of tariffs would not avoid the need for controls at the Irish border, because physical checks on standards of goods would be required if the UK leaves the EU single market.

In the same interview with the BBC, Mr Carney also said 60 per cent of UK businesses that export to the EU still do not have all the documents they need to continue exporting if Britain crashes out of the bloc.

“[That] means there are 150,000 businesses that don’t yet have some of the paperwork that’s required in order to export,” he said.

He added that firms had built up inventories so they could continue supplying UK customers in the event of no deal, but the stocks would last only “weeks”.