Theresa May rejected an EU offer designed to rescue a Brexit deal because it would add billions more to the £39bn “divorce bill”, The Independent has learned.

Brussels floated a one-year extension to the post-departure transition period, until the end of 2021, to make it less likely the Irish border “backstop” would be needed – and, therefore, easier for the UK to accept.

But an extension would require paying billions extra to the EU, when the exit bill has already angered Tory MPs, prompting the prime minister to stamp on the idea.

An EU source told The Independent that the UK’s response to the offer, made during last week’s aborted talks, suggested the “financial implications would be complicated”.

Even a short extension would add around £5bn to the divorce bill, MPs heard earlier this year – while the £39bn figure itself could rise if EU pension and project liabilities are higher.

The revelation underlines the huge difficulty Ms May faces in trying break the deadlock over the Irish border, which threatens to wreck her hopes for a Brexit agreement with the EU.

She admitted this week that Brussels had rejected her plan for the entire UK to remain in the EU’s customs territory temporarily, to avoid a return to a hard border in Ireland.

Instead, the EU is insisting Northern Ireland alone remains in the EU customs union, risking a border in the Irish Sea until a future trade deal and technology provide an alternative solution – which the prime minister has called “unacceptable”.

The longer transition is a fresh idea to resolve the impasse by creating more time for new solutions, making the “backstop” easier for the UK to sign up to.

Asked about an extension, a Downing Street spokesperson did not deny the proposal had been put forward, saying only: “That is not our position.”

Simon Coveney, Ireland’s deputy leader, confirmed the idea, saying the EU wanted to “ensure that the backstop is never likely to be used by creating the space and time”.

Boris Johnson 'suicide vest' Brexit jibe causes Tory divide Show all 14 1 /14 Boris Johnson 'suicide vest' Brexit jibe causes Tory divide Boris Johnson 'suicide vest' Brexit jibe causes Tory divide Internal divisions in the Conservative Party have exploded into a bitter public row over Boris Johnson‘s “disgusting” criticism of Theresa May. Some senior Tories furiously denounced the former foreign secretary after he accused the prime minister of having ”wrapped a suicide vest” around Britain Reuters Boris Johnson 'suicide vest' Brexit jibe causes Tory divide Sajid Javid, Home secretary Sajid Javid, the home secretary, rebuked his former cabinet colleague and said: “I think there are much better ways to articulate your differences.” He told the BBC’s Andrew Marr Show that the public wanted politicians to use “measured language” BBC/PA Boris Johnson 'suicide vest' Brexit jibe causes Tory divide But other MPs leapt to Mr Johnson’s defence, as dividing lines ahead of a possible leadership contest begin to take shape. The Uxbridge MP has repeatedly criticised Ms May’s Chequers plan and used a newspaper article on Sunday to suggest it amounted to “wrapping a suicide vest around the British constitution”. His latest salvo at the prime minister prompted immediate condemnation, with one minister publicly vowing to end Mr Johnson’s career over the matter PA Boris Johnson 'suicide vest' Brexit jibe causes Tory divide Alan Duncan, Foreign minister Alan Duncan, a foreign minister who worked in Mr Johnson’s team for two years, wrote on Twitter: “For Boris to say the PM’s view is like that of a suicide bomber is too much. This marks one of the most disgusting moments in modern British politics. “I’m sorry, but this is the political end of Boris Johnson. If it isn’t now, I will make sure it is later.” Getty Boris Johnson 'suicide vest' Brexit jibe causes Tory divide James Brokenshire, Housing secretary Housing secretary James Brokenshire added his voice to the criticism, calling Mr Johnson’s comments ”wrong” He said: “I think he is wrong on this...I think the tone that he has used isn’t right and I think that we just need to be very focused on actually moving forward with the Chequers plan.” AFP/Getty Boris Johnson 'suicide vest' Brexit jibe causes Tory divide Zac Goldsmith But as Tory hostilities spilled over into open public warfare, Richmond Park MP Zac Goldsmith, an ally of Mr Johnson, hit back at Mr Duncan. He wrote: “There are a number of possible motives behind this tweet, but given its author, we can be certain ‘principles’ aren’t one of them.” Getty Boris Johnson 'suicide vest' Brexit jibe causes Tory divide Jacob Rees-Mogg Senior Tory Brexiteer Jacob Rees-Mogg told The Independent he thought Mr Johnson’s “suicide belt” accusation was little more than “a characteristically colourful catchphrase”. He added: “I agree with the sentiment. The criticism of Boris’s wording merely serves to highlight his point. It means more people hear of Boris’s criticism of Chequers and many will agree with him.” Reuters Boris Johnson 'suicide vest' Brexit jibe causes Tory divide Nadine Dorries Nadine Dorries, another Brexit supporter, said Mr Johnson’s opponents were “terrified of his popular appeal”, adding: “Don’t underestimate the vitriol that’ll be directed towards Boris today. He delivered the Leave vote, Remainers and wannabe future PMs hate him.” If Mr Johnson became leader and prime minister he would deliver a “clean and prosperous” Brexit, she said Rex Boris Johnson 'suicide vest' Brexit jibe causes Tory divide Andrew Bridgen Andrew Bridgen said Ms May was to blame for her leadership problems. Asked if Mr Johnson had put a bomb under her leadership, Mr Bridgen said: “I think that Theresa May did that herself when she put forward the Chequers proposals without consulting widely prior to that.” Boris Johnson 'suicide vest' Brexit jibe causes Tory divide Steve Baker, former Brexit minister Steve Baker, the former Brexit minister, warned Ms May the Conservatives faced a "catastrophic split" if she did not jettison her Chequers plan. Mr Baker, who quit the government in July over the scheme, said: “When negotiating, the prime minister needs to demonstrate her intent and also her power to deliver. "If we come out of conference with her hoping to get Chequers through on the back of Labour votes, I think the EU negotiators would probably understand that if that were done, the Tory party would suffer the catastrophic split which thus far we have managed to avoid.” But he insisted he did not want a change in the Conservative leadership, saying Brexiteers did “not want to be in a position of conflict with our own prime minister” Reuters Boris Johnson 'suicide vest' Brexit jibe causes Tory divide Tom Tugendhat The deep divisions on the Tory benches were laid bare as Tom Tugendhat, who chairs the Commons foreign affairs committee and is a possible leadership rival to Mr Johnson, also hit out at the former foreign secretary. Recalling how he encountered a suicide bomber in Afghanistan during his time in the army, Mr Tugendhat told Mr Johnson to “grow up”. He said: “A suicide bomber murdered many in the courtyard of my office in Helmand. The carnage was disgusting, limbs and flesh hanging from trees and bushes. Brave men who stopped him killing me and others died in horrific pain. “Some need to grow up. Comparing the PM to that isn’t happy.” PA Boris Johnson 'suicide vest' Brexit jibe causes Tory divide Alistair Burt Alistair Burt, another Foreign Office minister who worked in Mr Johnson’s team, said: “I’m stunned at the nature of this attack. There is no justification for such an outrageous, inappropriate and hurtful analogy. “If we don’t stop his extraordinary use of language over Brexit, our country might never heal. Again, I say, enough.” AFP/Getty Boris Johnson 'suicide vest' Brexit jibe causes Tory divide It comes amid that Ms May’s former aides drew up a dossier on Mr Johnson’s sexual encounters with the aim of undermining his leadership prospects. The document was compiled in 2016, when the Uxbridge and South Ruislip MP was seen as the main rival to Ms May in her bid to enter No 10. Downing Street and Conservative Campaign Headquaters (CCHQ) both denied having leaked the 4,000 word memo after it was circulated around Westminster AFP/Getty Boris Johnson 'suicide vest' Brexit jibe causes Tory divide Mr Johnson confirmed that his 25-year marriage to wife Marina had ended AP

“We have always said that we are happy to show flexibility in terms of how we get to a destination where-by there is a backstop, or an insurance mechanism, in place to reassure people on the island of Ireland that they are not going to see the re-emergence of physical border infrastructure,” he said.

“The EU side, certainly, is willing to allow more time in the transition period to agree an alternative solution to a backstop.”

The proposal was put forward during the intense talks that ended with Dominic Raab, the Brexit secretary, returning empty-handed from Brussels on Sunday, after meeting Michel Barnier, the EU’s chief negotiator.

Plans for further talks between officials were abandoned – and the EU reported “no grounds for optimism” ahead of the Brussels summit getting underway on Wednesday evening.

The source said: “A one-year extension to the transition was suggested by Michel Barnier suggestion, not the UK.

“The financial implications [UK budget contributions] would be complicated. We don’t think it will fly.”

However, some doubt a longer transition would allow Ms May to wriggle off the hook of the Irish border – because the EU would still insist she signs up to the backstop.

The 21-month transition period is due to run from next March until the end of December 2020, but will only be implemented if a withdrawal deal is struck.

The transition will also seal the UK’s legal commitment to the divorce bill – about half of which is for outstanding obligations from EU membership, with the other half “subs” for the 21 months.

Pro-Brexit Tories remain angry, because the money must be paid even if talks after Brexit day fail to strike a favourable trade agreement.

Officials have also acknowledged that the figure of £39bn – to be paid in instalments for nearly 50 years – could go up.

Arriving in Brussels, ahead of make-or-break appeal to EU leaders to soften their stance, the prime minister ducked questions about the offer to extend the transition.