British Prime Minister Theresa May faces another showdown in the Commons tomorrow as the DUP are set to join Labour and other opposition parties in a bid to force the Government to publish its legal advice on Brexit.

The move could delay the crucial vote on May’s controversial Brexit plan.

The Telegraph today reports that Sir Keir Starmer, Labour’s shadow Brexit secretary, Nigel Dodds, the DUP’s Westminster leader, Tom Brake, the Liberal Democrats’ Brexit spokesman, and Stephen Gethins, the SNP’s Europe spokesman, will write a joint letter to John Bercow, the House of Commons Speaker.

The letter will insist that the Government is in contempt of the parliament for failing to publish the full Brexit legal advice from Geoffrey Cox, the Attorney General.

The calls will come amid reports that the advice contains a stark warning that, as part of this plan, the UK could be tied to the EU Customs Union indefinitely through the Northern Ireland backstop.

The row could delay the start of a marathon 40-hour debate set over five sitting days on the Brexit deal, starting on Tuesday.

The move risks eroding completely the trust between Tories and the DUP, whose 10 MPs are keeping May’s minority government in office.

Meanwhile, Michael Gove has insisted the Government can win the crucial Commons vote, despite scores of Tory MPs threatening to vote against it.

As MPs prepare to begin five days of debate ahead of the vote on December 11, the Environment Secretary acknowledged it would be “challenging” to get it through the House.

But while he accepted the deal was not “perfect”, he said the alternative was either “no deal or no Brexit”.

His warning came as shadow Brexit secretary Sir Keir Starmer said it was “inevitable” Labour would move a vote of no confidence in the Government if the deal was voted down.

Mr Gove, who was one of the leaders of the Leave campaign in the referendum in 2016, said he had reflected “long and hard” before deciding to back the plan.

But while there were aspects of the deal he found “uncomfortable”, he believed it was now the right way forward.

“I reflected long and hard about this deal but I concluded, like lots of people, that while it is imperfect it is the right thing to do,” he told BBC One’s The Andrew Marr Show.

“One of the things that I hope people will have the chance to do over the next nine days is to recognise that we should not make the perfect the enemy of the good.

“We have got to recognise that if we don’t vote for this, the alternatives are no deal or no Brexit.

“I believe that we can win the argument and win the vote. I know it is challenging.”

Downing Street will hope that the intervention of Mr Gove – who turned down the job of Brexit Secretary following the resignation of Dominic Raab – will help persuade some Brexiteers to back the agreement.

The Environment Secretary said the most difficult element of the deal was the Northern Ireland “backstop”, intended to prevent the return of a hard border with the Republic.

Brexiteers have warned it could see the UK tied to EU customs arrangements for years with no exit mechanism, while negotiations continue with Brussels on a trade deal.

But Mr Gove insisted there was no incentive for the EU to prolong Britain’s stay in the backstop.

“The critical thing about the backstop is, however uncomfortable it is for the UK, it is more uncomfortable for the European Union,” he said.

“We will have tariff-free access to their markets without paying a penny. And, more than that, we will have control of our borders.

“While it does contain elements that for a Unionist or for a Brexiteer aren’t perfect, it also contains elements that for any European politician would allow them to see Britain having a competitive advantage over their own country and their own economy.

“This fundamentally works against the interests of the single market and against the interests of European nations.”

He dismissed a claim by French president Emmanuel Macron that the EU would be able to exploit the backstop to extract concessions from Britain over access to fisheries.

“He doesn’t have us over a barrel. We have got him over a barrel of herring and a barrel of mackerel. He wants that access to our waters. We can sit in the backstop and say ‘No, absolutely not’,” he said.