Theresa May's top Brexit adviser has privately warned her that a key element of her exit blueprint would be a "bad outcome" for the UK.
Oliver Robbins, who led the behind-the-scenes talks with Brussels, reportedly told the prime minister that there was no legal guarantee that Britain could withdraw from the "backstop", which aims to prevent a hard border in Ireland if future trade talks fail.
His comments will enrage Brexiteers, who fear the plan will keep Britain closely tied to the EU after leaving the bloc, and will heap pressure on the prime minister as she battles to save her Brexit deal ahead of a Commons showdown on December 11.
In a letter to the prime minister, obtained by the Daily Telegraph, Mr Robbins said: "We should not forget that the backstop world, even with a UK-EU customs union, is a bad outcome with regulatory controls needed somewhere between GB and NI, serious and visible frictions and process between GB and the EU, and no security co-operation provided for."
Mr Robbins is said to have argued that extending the transition period would offer a more "cast iron escape route" than entering into the backstop, which will commence in December 2020, if a better solution cannot be found.
The senior civil servant faces a grilling by MPs on Monday, along with the newly-appointed Brexit secretary, Stephen Barclay, who is making his first appearance before the Brexit select committee.
The leak will also intensify demands for ministers to publish the full legal advice on the Brexit deal to avoid a major constitutional row.
Geoffrey Cox, the attorney general, will attempt to head off a threat by opposition parties to trigger contempt proceedings against the government over its refusal to publish the advice.
Cross-party MPs say they need to see the full advice - including Mr Cox's legal opinion - before they vote on the Brexit deal next week.
The government was ordered to publish the report after choosing not to oppose an arcane procedure, known as a humble address, over fears of an embarrassing Commons defeat.
Labour says it has the support of Ms May's DUP allies, as well as the SNP and the Liberal Democrats, while many Tory MPs are also demanding to see the advice.
Meanwhile, ex-foreign secretary Boris Johnson said it was a "scandal" that ministers were withholding the full advice.
"It is outrageous that the public should be prevented from knowing the full legal implications of this appalling deal - when it is their rights, their freedoms, their hard-won suffrage, that are about to be bartered away," he said.
Sir Keir Starmer, the shadow Brexit secretary, said the government was setting itself on a "collision course" with parliament, adding: "If it is now moving into the territory of ignoring parliament altogether and breaching an order of parliament, then we're getting to very, very deep water."
Anyone found in contempt of parliament can be expelled from the House. However it is more likely that a finding against the government would serve to weaken the prime minister's authority.
The row comes as MPs were gearing up for an explosive five-day debate on the Brexit deal, which will culminate in a meaningful vote on December 11 that could determine the future of Ms May and her government.
Labour has vowed to table a no-confidence motion if the deal is voted down, in an attempt to force the government to hold a general election.
The DUP is also considering abandoning the government if there was a confidence vote, despite their "confidence and supply" agreement which props up Ms May's administration.
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