BORIS Johnson has conceded that, while it is “epically likely” Britain will secure a comprehensive trade deal with the EU by December 2020, there is a small chance it will not.

During the General Election campaign, the Prime Minister expressed supreme confidence that a post-Brexit deal would be struck, claiming the chances of failure were “absolutely zero”.

But in his first post-election interview, he told the BBC it was “very, very, very likely” an agreement would be sealed but then conceded: “You always have to budget for a complete failure of common sense.”

Ursula von der Leyen, the new European Commission President, has argued that securing a deal, effectively within six months given the parliamentary ratification process needed across the continent, was a “very tight” deadline, meaning a deal was highly unlikely. She suggested without an extension all Brussels was likely to offer was a “bare-bones” trade deal on quotas and tariffs with a more detailed one in the future.

On Tuesday, Andrej Plenkovic, the Prime Minister of Croatia, which holds the rotating council presidency, told MEPs: "The transition period of 11 months is extremely short and it will take a lot of goodwill and a lot of work to bring these negotiations to an end in that period."

Later, EU27 governments at a meeting in Brussels stressed a regime of common standards covering such issues as employment law, state aid and environmental protection would have to be established.

In his interview, Mr Johnson insisted, after Brexit Day the UK would begin the process of negotiating new trade deals with countries outwith the EU.

“From January 31, what we're going to do is start working with our friends and partners around the world; not just with the EU. We're going to start building new relationships with friends and partners around the world."

Meanwhile, the PM suggested Britons could "bung a bob for a Big Ben bong" on Brexit night as it was estimated that reactivating the iconic clock in the Elizabeth Tower – currently being refurbished – to chime at 11pm on January 31 would cost taxpayers £500,000.

Mr Johnson explained how the Government was “working up a plan” but Downing St later made clear there was no specific Whitehall scheme.

The PM’s spokesman explained: "If the public wants Big Ben to bong and the money is raised, then that is great. We will make sure that - whatever happens in regard to Big Ben's bongs - January 31 is properly marked. It is a significant moment in our history."

Sir Lindsay Hoyle, the Commons Speaker, who chairs the parliamentary commission, noted that it would cost around £50,000 per bong if Big Ben were to chime at the end of the month.

He said: "We also have to bear in mind that the only people who will hear it will be those who live near or are visiting Westminster."

Tory backbencher and high profile Brexiteer, Mark Francois, said he would be prepared to chip in £1,000 to hear the iconic clock chime in Brexit, saying: “I'm very confident we can raise a half a million pounds in a few days.”