The Democratic Unionist Party will join Labour and other opposition parties on Monday in a bid to force the Government to publish its legal advice on Brexit – a move that could delay the crucial vote on Theresa May’s plan.

In an explosive alliance that will rock the Government, 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 Parliament for failing to publish the full Brexit legal advice from Geoffrey Cox, the Attorney General, that underpins Mrs May’s Brexit deal, and call for an urgent debate to resolve the constitutional row. Eurosceptics believe that the advice will warn that the UK cannot escape the EU customs union after Brexit. The row could delay the start of a marathon 40-hour debate set over five sitting days on the Brexit deal, starting on Tuesday.

One senior Cabinet minister claimed that Mr Bercow “will try to thwart Brexit in any way that he can”.

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