The group behind the successful Article 50 battle in January are reportedly prepared to return to the courts if the Government refuse to enshrine the parliament vote into law.
Gina Miller, who was key to January’s Supreme Court victory, is said to be part of the group poised for another legal battle as the Brexit process threatens to drag on long past the March 2019 deadline.
Ministers fear allowing MPs to amend the final EU deal could serious impact the Brexit process, with fears emerging infighting in Britain could leave Brussels to believe they have the upper hand.
A source close to the original case told the Times: “Unless parliament is given a binding vote in legislation on the withdrawal agreement then I would expect a number of challenges.
“The Article 50 case made clear that it is all about respecting parliament’s sovereignty.”
In another blow to Mrs May, senior legal figures believe Mrs Miller and her allies are likely to be successful if they take the case to court – because it would be based on the same principles as the successful Article 50 case.
Lord Hope of Craighead warned: “Without the explicit authorisation of parliament in legislation [the withdrawal agreement] is vulnerable.”
The EU withdrawal bill is set to return to the Commons next month and challenges to the Government’s refusal to enshrine the withdrawal agreement in law are expected.
Ten Conservative MPs, including former attorney-general Dominic Grieve, have signed an amendment saying that Brexit must be “subject to the prior enactment of a statute by parliament approving the final terms of withdrawal”.
Meanwhile, Nicky Morgan, the former cabinet minister, insisted Tory rebels are “deadly serious” about voting against the Government to ensure MPs get a say on the final Brexit deal.
She said: “Reports have reached members on this side that the Secretary of State doesn’t think that those Conservative members who have signed that amendment are serious about supporting it if we need to.
“Can I tell him we are deadly serious and it would be better for the Government to attempt a concession strategy on having a withdrawal agreement secured by statute sooner rather than later for all concerned.”
Brexit Secretary David Davis was forced to backtrack this week after initially suggesting MPs may not get to vote on the deal until after Britain has left the EU.
In PMQs on Wednesday, the Prime Minister contradicted Mr Davis’ comments and said she was confident MPs would get a vote before Britain left the bloc.
This prompted the Brexit Secretary to issue a statement confirming he “expects and intends” for there to be a vote in Parliament before Britain leaves.