Theresa May’s hopes of avoiding defeat on her Brexit deal hinge on convincing MPs that she has secured substantial “legally binding” changes to ensure the Irish backstop cannot be permanent.

The Prime Minister reached an 11th-hour agreement with the European Union in Strasbourg late on Monday night and said she “passionately believed” it addressed concerns raised by MPs who feared the backstop would keep the UK in a customs arrangement with the EU indefinitely.

But the fate of the deal hangs on legal scrutiny by Attorney General Geoffrey Cox and a “Star Chamber” of Brexiteer lawyers convened by the European Research Group (ERG) of MPs.

At a late-night joint press conference with European Commission president Jean-Claude Juncker, Mrs May said the three new documents agreed provided the legal assurances critics of her stance had called for.

Mrs May needs to win over scores of Tory MPs if she is to have any hope of reversing the 230-vote defeat she suffered when the Commons considered her Brexit deal in January.

The PM said: “What we have secured is very clearly that the backstop cannot be indefinite. Cannot become permanent. It is only temporary. If it is the case that we were ever to get into the backstop.”

Mrs May added: “Now is the time to come together to back this improved Brexit deal and to deliver on the instruction of the British people.”

But shadow Brexit secretary Sir Keir Starmer – an eminent lawyer –  said he would be “surprised” if the Attorney General was able to change his advice on the deal.

Mr Cox’s advice on the previous version of the deal could not rule out being trapped “indefinitely” in the backstop, the set of measures which would tie the UK closely to Brussels’ rules and tariffs in order to avoid a hard border with Ireland.

Sir Keir, who called for Mr Cox to face MPs in the Commons, said: “Having studied the documents, I would be surprised if they are sufficient to enable the Attorney General to change the central plank of his December legal advice.”

The changes did not include “a mechanism for unilateral exit from – or termination of – the backstop”.

Meanwhile, Mr Cox dismissed as “bollocks” a suggestion that he had been pressured into finding a way to change his legal advice.

Tory Jacob Rees-Mogg, chairman of the influential European Research Group of Brexiteer MPs, said the “Star Chamber” of lawyers assembled by the group – including DUP MP Nigel Dodds, Conservative veteran Sir Bill Cash and outside experts – was also examining the deal.

“My focus will be on whether the unilateral declaration is genuinely unilateral,” he told BBC Radio 4’s Today programme.

He added that “many Conservatives will be heavily influenced by the DUP’s view”.

On a day of high drama in Westminster, the Prime Minister was chairing Cabinet from 9.30am before heading to the Commons to open the debate on her revised Brexit package.

MPs are expected to vote at 7pm, with Environment Secretary Michael Gove saying it is “make your mind up time”.

European Commission president Mr Juncker insisted there would be no further negotiations on the issue.

Mr Juncker said: “In politics, sometimes you get a second chance. It is what we do with the second chance that counts. Because there will be no third chance.”

Ireland’s Taoiseach Leo Varadkar said: “The further text agreed yesterday provided additional clarity, reassurance and guarantees sought by some to eliminate doubts or fears of some, however unreal, that the goal was to trap the UK indefinitely in the backstop.

“It is not – these doubts and fears can be put to bed.”

Dutch Prime Minister Mark Rutte said he hoped MPs would back the deal as “there is no alternative”.

Mrs May believes the three new documents give MPs the legally-binding reassurances they require to approve her Withdrawal Agreement and Political Declaration on the future EU/UK relationship.

The package involves “the legally binding joint instrument” relating to the Withdrawal Agreement.

The Government states that the document “reduces the risk that the UK could be deliberately held in the Northern Ireland backstop indefinitely and commits the UK and EU to work to replace the backstop with alternative arrangements by December 2020”.

The second new document is a “unilateral declaration by the UK” which sets out “the sovereign action the UK would take to provide assurance that the backstop would only be applied temporarily”.

The final document is a supplement to the Political Declaration “setting out commitments by the UK and the EU to expedite the negotiation and bringing into force of their future relationship”.

If the package passes the Commons, leaders of the 27 remaining EU states will be asked to endorse the new documents at a scheduled European Council summit in Brussels on March 21-22, before the final step of ratification by the European Parliament.

The DUP, whose 10 MPs prop up Mrs May’s administration, has pledged to carefully analyse the Brexit deal add-ons, remaining non-committal on whether it would now back the Withdrawal Agreement.

Deputy leader Mr Dodds told the Commons “all of this will need to be taken together and analysed very carefully”.

Labour leader Jeremy Corbyn has called on MPs to reject the deal and accused Mrs May of a plan to “recklessly run down the clock” before March 29.