Conservative MPs have attacked the government after a paper setting out official legal advice on Theresa May’s Brexit deal, confirmed Britain cannot leave the arrangement it creates without the EU’s agreement.

The command paper which summarises the advice given to the cabinet from the attorney general, says that the controversial Irish ‘backstop’ will apply “unless and until” it is superseded by alternative arrangements agreed between Brussels and London.

A further section of the document states clearly that “it is not possible under international law” for the UK to escape the withdrawal agreement unilaterally once it is in force.

The summary has been released by the government in an attempt to assuage a motion passed by the Commons that the full legal advice relating to Ms May’s deal be published – something ministers have refused to do.

But even in the summary there are elements that Brexiteers criticised, believing that it has confirmed their worst fears about the deal.

Nadine Dorries MP said: “No MP with a shred of respect for the democratic process can support this deal.”

Speaking to her followers on Twitter she added: “We now know as a result of the legal advice, that those being told it can all be changed later, we just need to get the deal over the line, are being lied to. We cannot change anything without breaking the law and we Brits don’t do that. We play by the rules.”

Jacob Rees-Mogg, who chairs the anti-Brexit European Research Group, said the paper “exposes inaccurate government spin on the proposed Brexit deal”.

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The most controversial issue in the withdrawal agreement is over the ‘backstop’ agreement, to set out what happens to keep the Irish border open if no trade deal is secured by the end of the Brexit transition period in December 2020.

Under the plan the whole UK would stay in a customs union, but the legal document released on Monday confirms that this set up would continue to apply “unless and until it is superseded” by a subsequent agreement.

It later goes on: “The agreement does not contain any provision on its termination.

“In the absence of such a provision, it is not possible under international law for a party to withdraw from the agreement unilaterally.”

The advice also confirms that the UK faces making additional payments to Brussels if the Brexit transition period is extended.

Under the terms of the withdrawal agreement it is due to run until the end of December 2020 but can be extended by up to two years if both sides agree.