Theresa May is facing a fresh legal challenge to her Brexit plans as campaigners prepare to petition the High Court for Britain to stay in the single market.

The claimants, including one Remain and one Leave voter, are seeking a judicial review into Ms May's apparent decision to opt for a hard Brexit – meaning foregoing access to the single market in exchange for withdrawing Britain from freedom of movement for European workers.

Such a review could theoretically give MPs the power of veto over the terms of the UK's exit.

Peter Wilding, chairman of the pro-Europe pressure group British Influence who is credited with coining the word Brexit, and Adrian Yalland, a Conservative lobbyist who voted to leave, launched the High Court bid to ensure the country got a “win-win, smart Brexit; not a lose-lose ideological hard Brexit which will damage the UK”.

They told The Sunday Times there was no mandate for leaving the single market and were concerned the country would be acting illegally if it did not get a proper mandate.

Mr Yalland, whose partner is Polish, said: “The Government thinks we leave the single market ‘automatically’ when we leave the EU. I think they could be wrong.

“So my judicial review will ask the court to decide who is correct.

"Why does this matter? Well, because membership of the single market is the ‘ace’ card which the EU obviously wants in its hand during the Brexit negotiations.

Supreme Court Brexit Challenge Show all 13 1 /13 Supreme Court Brexit Challenge Supreme Court Brexit Challenge People wait to enter the public gallery outside the Supreme Court ahead of the challenge against a court ruling that Theresa May's government requires parliamentary approval to start the process of leaving the European Union, in Parliament Square, central London Reuters Supreme Court Brexit Challenge Gina Miller, co-founder of investment fund SCM Private arrives at the Supreme court in London on the first day of a four-day hearing Getty Supreme Court Brexit Challenge A man waves the EU flag in front of the Supreme Court Getty Supreme Court Brexit Challenge Satirical artist Kaya Mar poses with two of his paintings in front of the Supreme Court Getty Supreme Court Brexit Challenge Pro-Europe protestors dressed as Supreme Court Justices stand outside the Supreme Court ahead of the first day of a hearing into whether Parliament's consent is required before the Brexit process can begin. The eleven Supreme Court Justices will hear the government's appeal, following the High Court's recent decision that only Parliament can trigger Article 50 Getty Supreme Court Brexit Challenge The eleven Supreme Court Justices will hear the government's appeal, following the High Court's recent decision that only Parliament can trigger Article 50 Getty Supreme Court Brexit Challenge Businesswoman Gina Miller arrives at the Supreme Court ahead of the first day of a hearing into whether Parliament's consent is required before the Brexit process can begin Getty Supreme Court Brexit Challenge Attorney General Jeremy Wright arrives at the Supreme Court in London EPA Supreme Court Brexit Challenge Protesters outside the Supreme Court in London, where the Government is appealing against a ruling that the Prime Minister must seek MPs' approval to trigger the process of taking Britain out of the European Union PA wire Supreme Court Brexit Challenge A protesters wearing a judge's wigs and robes stands outside the Supreme Court ahead of the challenge against a court ruling that Theresa May's government requires parliamentary approval to start the process of leaving the European Union, in Parliament Square, central London Reuters Supreme Court Brexit Challenge A protester holds up a placard outside the Supreme Court ahead of the challenge against a court ruling that Theresa May's government requires parliamentary approval to start the process of leaving the European Union, in Parliament Square, central London Reuters Supreme Court Brexit Challenge Pro-Europe protestors dressed as Supreme Court Justices stand outside the Supreme Court Getty Supreme Court Brexit Challenge A man waiting to enter the public gallery waves a European Union flag outside the Supreme Court ahead of the challenge against a court ruling that Theresa May's government requires parliamentary approval to start the process of leaving the European Union, in Parliament Square, central London Reuters

“But if the UK can trigger Brexit and not be forced out of the single market then the ‘ace’ card is actually in the UK's hands, which significantly enhances the UK's Brexit negotiating position.”

He said if the courts rule the Government is wrong about single market membership, they must also trigger Article 127 of the Lisbon Treaty which regulates it.

If they do so, he argues, they may need to get the permission of Parliament as membership was originally ratified there in 1993.

It comes as the Government faces being forced to get Parliament's approval to trigger Article 50 after the Supreme Court heard arguments from campaigners Gina Miller and Deir Dos Santos this week.

They argue the Government cannot take away British citizens' rights without the consent of Parliament, while the Government argues it alone has the right to decide on foreign policy matters.