The UK Government has been forced to hand over legal documents detailing why it is refusing to let MPs have a vote to decide when and how Britain should leave the European Union.

High Court judge Mr Justice Cranston made the decision ahead of a crunch hearing next month brought by "concerned citizens," including the People’s Challenge group, who claim Theresa May should not be able to enact Brexit without Westminster’s approval.

The legal challenge has been described as the most important constitutional law case in living memory.

Nicola Sturgeon, who wants the Scottish Parliament also to have a vote on the UK’s Brexit proposal, has made clear she is keeping a “close eye” on the legal case but suggested the matter of whether or not MPs and MSPs should have a vote “probably will end up in the Supreme Court,” the UK’s final arbiter on important constitutional matters.

Such is the importance of the case, the UK Government announced it would be represented in court by its top legal officer, the Attorney General Jeremy Wright.

"The country voted to leave the European Union in a referendum approved by Act of Parliament. There must be no attempts to remain inside the EU, no attempts to rejoin it through the back door and no second referendum,” declared Mr Wright.

"We do not believe this case has legal merit. The result of the referendum should be respected and the Government intends to do just that," he added.

Bindmans, a law firm representing some of those mounting the legal challenge, said Government lawyers had wanted to keep their legal arguments secret ahead of the court case but were forced to make them public following the ruling.

In released documents, lawyers for Brexit Secretary David Davis insisted it was "crystal clear" that the UK Government had the power to enact withdrawal from the EU; the decision to leave the EU should not be subject to the courts but was "a matter of the highest policy reserved to the Crown".

But John Halford for Bindmans said: "The court's order allows a floodlight to be shone on the Government's secret reasons for believing it alone can bring about Brexit without any meaningful parliamentary scrutiny."

It is thought whoever loses in the High Court will appeal and this could go straight for a ruling by the Supreme Court.