Britain could be trapped "indefinitely" in a customs union with the European Union if MPs back Prime Minister Theresa May’s Brexit deal, according to details of the government's full legal advice on the deal.

The legal advice was published this morning following yesterday’s parliamentary vote, which found the government in contempt of parliament.

"Despite statements in the Protocol that it is not intended to be permanent and the clear intention of the parties that it should be replaced by alternative, permanent arrangements, in international law the Protocol would endure indefinitely until a superseding agreement took its place," the advice said.

The protocol refers to the so-called backstop agreement in Mrs May's withdrawal deal she has agreed with the EU.

The backstop is designed to avoid a hard border on the island of Ireland post-Brexit.

Meanwhile the DUP, which is propping up Mrs May’s government, said that the British government has broken its promise to the party.

It also said that the government has put pressure on the arrangement that keeps Mrs May in power. 

The DUP is angry over the backstop arrangement, saying it essentially splits Northern Ireland from the rest of mainland Britain. 

The party's Brexit spokesman Sammy Wilson told the British parliament: "We had an arrangement to keep the government in power ... promises were made".

He said Mrs May had promised his party that Northern Ireland would get a final say in whether such a backstop was put in place.

Mr Wilson said: "There has been bad faith, the agreement and the understanding we had has been broken ... that has caused tension and going down this road will create further tensions.

"We want to see the agreement honoured because we want to see the United Kingdom preserved."

Eurosceptic MP Jacob Rees-Mogg has said that the DUP will vote against any attempt to topple Mrs May if she is defeated in a House of Commons vote on her Brexit deal.

In a statement, the European Research Group said: "The DUP will support the government in a confidence motion if the Withdrawal  Agreement is voted down.

"But the risk of losing (their support) and having an election is if the WA goes through."

The ERG statement was issued following a meeting with DUP deputy leader Nigel Dodds.

More Brexit stories

Earlier, UK Labour’s Brexit spokesperson Keir Starmer said the full legal advice on Brexit reveals "central weaknesses in the government’s deal".

Mr Starmer in a post on Twitter said: "Having reviewed the Attorney General's legal advice, it's obvious why this needed to be placed in the public domain.

"All week we have heard from government ministers that releasing this information could harm the national interest. Nothing of the sort. All this advice reveals is the central weaknesses in the government's deal.

"It is unthinkable that the government tried to keep this information from parliament - and indeed the public - before next week's vote."

Green MP Caroline Lucas tweeted sections of the legal advice, which she said suggested it was not received by the cabinet until 13 November - the day MPs first voted for it to be released.

Ms Lucas highlighted concerns in the document that the protocol setting out the backstop arrangements for Ireland would "endure indefinitely".

According to the extracts, she said, a review mechanism in the Brexit deal "does not provide a unilateral route out of the backstop" and there is "a legal risk that UK could become stuck in protracted and repeating rounds of negotiations".

In its conclusion, it says: "In the absence of a right of termination, there is a legal risk that the United Kingdom might become subject to protracted and repeating rounds of negotiations.

"This risk must be weighed against the political and economic imperative on both sides to reach an agreement that constitutes a politically stable and permanent basis for their future relationship.

"This is a political decision for the government."

The finding of contempt of parliament came ahead of a crucial vote next week by MPs on whether or not to accept Mrs May's Brexit plan.

Mrs May also suffered a defeat on a vote that will see MPs get a bigger say on what happens next if her deal is rejected.

Additional reporting PA