BRITAIN would be locked “indefinitely” into the EU’s customs union if Theresa May’s Brexit deal proposal was passed, leaked documents of the attorney-general’s legal advice claim.

The details have been suppressed with senior ministers saying the Prime Minister has refused to publish the advice because it highlights that the UK could end up being trapped in a “backstop” arrangement with Brussels.

4 The Prime Minister could hit a hurdle with her Brexit plans over the Attorney General's legal advice Credit: AP:Associated Press

The Sunday Times claims it has seen the letter Geoffrey Cox wrote to cabinet ministers last month.

It says: “The protocol would endure indefinitely.”

The country’s top legal officer added that the only way the UK could get out of the backstop would be to sign a new trade agreement, which could take years.

But he said Britain would only stay trapped if those talks collapsed.

4 Raab said the EU had a clear veto Credit: AFP or licensors

The letter was said to be so sensitive that ministers were given numbered copies to read which they were not allowed to take from the room afterwards.

Labour said it is ready to combine with other opposition parties to start proceedings for contempt of Parliament unless the legal opinion of Attorney General Geoffrey Cox is published in full.

The DUP - which props up the Conservative government in the Commons - was said to be ready to sign a joint letter with other parties to Speaker John Bercow on Monday unless ministers back down.

It potentially represents another hurdle for Theresa May to overcome as she struggles to win backing for her deal in the crucial commons vote on December 11.

4 Geoffrey Cox's letter to cabinet ministers could prove a stumbling block for the Prime Minister Credit: Alamy Live News

MPs across Parliament have angrily accused ministers of ignoring the will of the House after they said only that they would release a "full reasoned political statement" on the legal position.

It follows a binding Commons vote last month requiring the Government to lay before Parliament "any legal advice in full" - including that given by the Attorney General - relating to the Withdrawal Agreement.

Former Brexit secretary Dominic Raab - who quit last month over the withdrawal agreement - said the legal position was clear.

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"The backstop will last indefinitely until it is superseded by the treaty setting out our future relationship, unless the EU allows us to exit," he told The Sunday Times.

"The EU has a clear veto, even if the future negotiations stretch on for many years, or even if they break down and there is no realistic likelihood of us reaching agreement.

"That's my view as a former international lawyer, but it is consistent if not identical with all of the formal advice I received."

Ministers have argued the legal advice is privileged, in the same way as any advice given by a lawyer to their client, and that government cannot function if it is required to release such confidential material.

4 Labour's Brexit secretary is said to be ready to join with other parties calling for the advice to be published in full Credit: London News Pictures

However, writing in The Sunday Telegraph, shadow Brexit secretary Sir Keir Starmer said it was essential MPs understood the "full legal implications" before they voted on the agreement.

"If the full legal advice is not forthcoming, we will have no alternative but to start proceedings for contempt of Parliament - and we will work with all parties to take this forward," he said.

"If ministers stubbornly refuse to obey the order of MPs then they risk triggering a historic constitutional row that puts Parliament in direct conflict with the executive.

"Although I accept the long-standing convention that Cabinet legal advice should be kept confidential, it's well-established that in exceptional circumstances that convention does not apply. And these are exceptional circumstances."

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Labour sources said that Sir Keir was ready to sign a joint letter with the DUP's Westminster leader Nigel Dodds, Liberal Democrat Brexit spokesman Tom Brake and SNP Europe spokesman Stephen Gethins, asking Mr Bercow to allow a motion "that the Government has held Parliament in contempt".

Under Commons rules, if the Speaker allows the motion to go before the House and the vote is carried, it would then be referred to the Committee of Privileges which would rule on whether a contempt of Parliament had taken place.

If it is decided that a contempt had occurred, the committee can recommend a suitable punishment which is then put back to MPs to agree.

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In theory, the most severe penalty is expulsion from the House, although the prospects of that happening would appear remote.

However any finding against the Government would be potentially highly damaging for Mrs May at a time when she is at her most vulnerable politically.

The news comes after the Sunday Express revealed that Britain would be unable to make trade agreements with the rest of the world unless they backed May’s plans.

Senior figures in government told the paper if MPs threw out the Prime Minister’s deal then the government was likely to put down an amendment that keeps Britain under Brussels rule in its customs union.