Theresa May under siege as she tries to sell her Brexit plan

READ MORE: Bitter Brexit battle breaks out as Nicola Sturgeon accuses Theresa May of 'selling out' Scotland's fishermen 

But she suffered a wave of criticism from senior Brexiteer Tories as it took nearly 40 minutes in the Commons chamber before there was any support from the Conservative benches.

Boris Johnson, the former Foreign Secretary, urged Mrs May to “junk the backstop,”claiming the divorce settlement made "a complete nonsense of Brexit".

Dominic Raab, the former Brexit Secretary, told the PM the “inescapable reality” was that her plan gave even more power away to Brussels.

Iain Duncan Smith, the former Tory leader, said he appreciated "enormously" Mrs May’s endeavours to deal with the Northern Irish backstop issue but noted: “None of this is at all workable unless we get the Withdrawal Agreement now amended, so that any arrangements we make strip out that backstop and leave us with that positive open border that we talked about."

Other Brexiteer MPs went further describing the agreement as a "betrayal" of the referendum, which amounted to "surrender".

Sir Jeffrey Donaldson for the Democratic Unionists, referring to possible technological solutions to keep an open border with the Irish Republic, said: “I say to the Prime Minister that if she wants to have the support of my party for the Withdrawal Agreement, we need to see an end of the backstop and those alternative arrangements put in place.”

READ MORE: Letters: Only through independence can we safeguard our rights after Brexit 

Later, Damian Green, Mrs May’s former de facto deputy, raised loyalist Tory cheers when he said: “Outside this House there is a much higher appreciation of the tenacity of the Prime Minister in pursuing a successful deal than we sometimes hear inside it.”

The PM, who will face a further grilling on Brexit by senior MPs next Thursday when she appears before the Commons Liaison Committee, told MPs the talks with Brussels were now at a “critical moment” ahead of Sunday’s special EU summit.

“All our efforts must be focused on working with our European partners to bring this process to a final conclusion in the interests of all our people,” she insisted.

Mrs May went on: “The British people want Brexit to be settled. They want a good deal that sets us on a course for a brighter future and they want us to come together as a country and to move on to focus on the big issues at home, like our NHS.

“The deal that will enable us to do this is now within our grasp. In these crucial 72 hours ahead, I will do everything possible to deliver it for the British people,” she declared.

But Jeremy Corbyn dismissed the text on Britain’s future relationship with the EU as “26 pages of waffle”.

The Labour leader said: “Nineteen extra pages but nothing has changed. The only certainty contained within these pages is that the transition period will have to be extended or we will end up with a backstop and no exit.

"It represents the worst of all worlds, no say over the rules that will continue to apply and no certainty to the future."

Earlier, Ms Sturgeon dismissed the text of the proposed future political agreement between the UK and the EU post Brexit, saying it amounted to "lots of unicorns taking the place of facts" and claimed it was “yet another Tory sell-out of Scotland’s fishermen”.

The accusation was echoed by her colleague Ian Blackford, the SNP leader at Westminster, when he told the PM that Scotland's fishing rights had been "thrown overboard as if they were discarded fish".

Declaring: "So much for taking back control, more like trading away Scotland's interests,” the Highland MP suggested Mrs May had just lost the votes of her Scottish Conservative colleagues, who could “not possibly vote in favour of this sell-out of Scottish fishing interests".

But the PM hit back, insisting the UK would become an "independent coastal state with control over our waters" before saying to Mr Blackford: "I will tell him what a sell-out of Scottish fishermen would be: it's the policy of the Scottish National Party to stay in the Common Fisheries Policy."