Jeremy Hunt has urged warring Tories to get behind Theresa May’s Brexit strategy as the Prime minister faced a fierce backlash against the idea of extending the UK’s transition period.

The Foreign Secretary said an EU proposal to continue the implementation process “might help” get an agreement on the future relationship and rejected claims that it amounted to a “capitulation”.

His plea for unity came as Mrs May suffered a hail of criticism from all wings of the Tory party, with one MP going as far as branding the leadership a “shit show”.

Meanwhile, the EU’s chief negotiator Michel Barnier warned that the UK’s withdrawal agreement could still fail over the issue of the Irish border, leading to an “extremely serious” no-deal Brexit.

Speaking to France-Inter radio, Mr Barnier said the deal was “90%” done and he was hoping to complete it “in a few weeks or a few months, as soon as possible”.

But asked if he was convinced an agreement would be reached, he replied: “I have no deep conviction on this subject, because in the UK the political situation is very complex and I don’t know what decisions Theresa May will take.”

Asked if the whole deal could founder over the border issue, he replied: “My answer is yes.”

Mr Barnier said that he believed the Brexit vote was fuelled in part by “nostalgia” as well as popular anger over the impact of globalisation, but said he had yet to find anyone who could explain how it would provide any answers to voters’ anxieties.

The firestorm around Mrs May came as she signalled she is ready to delay the UK’s final departure from the EU’s hold until 2021 in a last ditch bid to end the deadlock over the Irish border issue.

Britain has secured a 21-month transition period following the formal date of Brexit in March 2019, to give authorities and companies time to prepare for new arrangements. But EU chiefs have indicated this could be extended at the European Council summit this week in Brussels.

Brexit graphic
(PA Graphics)

Speaking to BBC Radio 4’s Today programme, the Foreign Secretary said: “The reason why this week has been difficult is because Theresa May has not buckled.

“She has held firm. She has stuck to her principles.

“It is precisely because she has not capitulated that we have not concluded this agreement.

“The one thing I would say to my colleagues is that the great strength of the EU in these negotiations is that the 27 EU nations have remained united.

“We now need to do the same behind Theresa May to maximise her negotiating leverage in Brussels and make sure she does come back with that deal that honours the letter and spirit of the referendum decision.”

Brexit
Theresa May also met with Taoiseach Leo Varadkar while in Brussels, to discuss the Irish border issue (Irish Government/PA)

But signs of unity appeared to be in short supply.

Brexiteer former party leader Iain Duncan Smith told Newsnight that a transition extension would see Britain pay “tens of billions of pounds” extra to the EU.

Former Remainer Nick Boles was also critical, saying it was “worse than being a member of the EU” because the UK would have to follow its rules and pay but have no vote.

The starkest language came from vocal backbencher and former soldier Johnny Mercer, who used an interview with The House magazine to open fire on the leadership.

Calling it a “shit show” he warned that if Tory internecine rows over Brexit let in Jeremy Corbyn “I don’t think we’d be forgiven for a generation and we wouldn’t deserve to be”.

Mr Hunt told Today the transition was not the main issue that needed to be resolved, which remained the so-called Northern Ireland “backstop” intended to ensure there was no return of a “hard border” with the Republic.

“The substantive area of disagreement is whether we would agree to a backstop which would allow for customs barriers down the Irish Sea – the effective break-up of the United Kingdom – or whether we agree to a backstop which would allow the UK as a whole to stay in the customs union indefinitely,” he said.

“Neither of those are acceptable. Those are two very important matters of principle for the United Kingdom.”

The Foreign Secretary said that Britain and the EU were going through a “pain barrier” in negotiations.

Mr Hunt told LBC: “In any negotiation… there comes a moment when you’re looking into the whites of the other person’s eyes, you’re understanding what each other’s bottom lines are and you need to go through, if you like, that pain barrier, and then you come out the other side with a deal that hopefully both sides can live with.”

Jacob Rees-Mogg, the leader of the pro-Brexit Tory European Research Group, poured scorn on Mr Hunt’s claim the Prime Minister was standing firm in the face of EU demands.

“Are you really telling me standing firm on the integrity of the United Kingdom is a really brave thing for the British Government to do? You would have thought this was to be taken as read,” he told the Today programme.

“Surely no government would ever go into a negotiation and agree to carve up the country for the sake of an international organisation.”

He warned any attempt to extend the transition period – with the UK continuing to pay billions into the EU on top of the £39 billion “divorce” bill – would be voted down by MPs.

“If the Government is saying to us we will pay £39 billion plus, for the extension, £15 or £16 billion more per annum and we don’t have anything in return other than a waffly political declaration, I think that will be very hard to get through the House of Commons,” he said.