Theresa May is facing fresh calls to deliver what will be seen as a “hard Brexit” from a group of 60 Conservative MPs.
Cameron-era ministers Michael Gove, Iain Duncan Smith, John Whittingdale and Theresa Villiers have joined the call to pull Britain out of the European single market and the customs union.
They claim getting out of the single market free trade zone is crucial to ensuring that the UK is not bound by Brussels regulations.
And they say pulling out of the tariff-free customs union is the only way to strike trade deals with countries outside it.
Eleven Labour, DUP and Ukip MPs also reportedly backed the call.
Coinciding with the launch of the European Research Group, a Tory pro-Brexit group, the MPs all backed the statement: “The UK must leave the European Economic Area (EEA) and the Customs Union.”
Tory MP Steve Baker, who chairs the group, said: “A vote to remain in the EEA or the Customs Union is a vote to be powerless over trade and domestic regulation and therefore poorer than we otherwise can be.
“The UK is ideally positioned to catalyse a new global trading system which works for everyone by promoting free and fair trade and defending against predatory practices.
“If we remain in the EEA or Customs Union, we will squander a multi-generational opportunity to relight the torch of liberty and prosperity. I am proud we are standing today against the wasted opportunity and fearfulness which would be to remain within the EU internal market.”
It came after a prominent Brexit supporter joined senior Tories in urging Mrs May to scrap an appeal against a court ruling which states that Parliament must approve the triggering of the formal leaving process.
Former cabinet minister Owen Paterson joined Sir Oliver Letwin, former attorney general Dominic Grieve, ex-solicitor general Sir Edward Garnier and former education secretary Nicky Morgan in calling for the PM to abandon her Supreme Court appeal.
The Scottish and Welsh governments have also been given permission to intervene in the case, in which the Government is appealing against a High Court ruling which states that Mrs May must get MPs’ permission before triggering the Article 50 exit process.
Sir Oliver, a former minister who was director of the Government’s “Brexit Unit” following the referendum, said that avoiding the Supreme Court would avert the risk of judges granting “veto powers” to the Scottish and Welsh devolved administrations.
Brexit-backer Mr Paterson said Mrs May should concede defeat because uncertainty is bad for the economy.
He said Article 50 should be triggered on time - the PM has promised it by April - to make Brexit an issue in French, German and Dutch elections next year in the hope that businesses in those countries will lobby for commitments to favourable trade terms with the UK.
Mr Paterson said: “I’m not a lawyer and I’m not an expert on this but - I wouldn’t have a bet on the Government winning this one.
“I do agree with Edward [Garnier] - it is not good to have a confrontation with the courts.
“Although I do think, you go right back to Article 9 of the Bill of Rights 1689, we have a very, very clear case.
“My concern is we deliver on what the people wanted.”
Three High Court judges ruled on 3 November that Mrs May does not have the power to trigger Article 50 alone. The Government quickly announced that it would appeal against the decision.
A Department for Exiting the European Union spokesman said: “We will robustly defend our position in the forthcoming appeal. As the Prime Minister made clear, our work is on track and we remain committed to triggering Article 50 by the end of March next year.”
Former Scottish first minister Alex Salmond said the Tories’ comments showed their contempt for Scotland’s rights.
The SNP’s Europe spokesman said: “Sir Oliver Letwin and co have now blurted out the true blue Tory attitude towards Scotland. They would even rather abandon the Brexit appeal than take any chance of the Supreme Court being sensitive to Scottish-European concerns.
“Every Tory in Scotland should cringe with embarrassment as the Tory contempt for Scotland’s rights as a nation has been laid bare.
“The trouble for the Tories in Scotland is that their Westminster colleagues have been caught telling the truth about their anti-Scottish prejudices.”