Campaigner Gina Miller is launching legal action to stop Boris Johnson suspending parliament in order to force through EU withdrawal without a deal.

The businesswoman’s lawyers have written to the probable next prime minister warning it would be “constitutionally unacceptable” and unlawful for him to lock MPs out of the Commons to stop them from blocking a no-deal outcome.

She has assembled the same legal team that successfully forced Theresa May in early 2017 to grant MPs a vote before triggering the Article 50 process that set the clock ticking on the UK’s exit from the EU.

The move comes days after former prime minister Sir John Major warned he would seek judicial review if Mr Johnson advised the Queen to use a mechanism known as “prorogation” to close the doors of Westminster.

The Tory leadership frontrunner is promising to take the UK out of the EU by 31 October, with or without a deal, if he defeats Jeremy Hunt in the race to succeed Ms May and has not ruled out prorogation to achieve it.

But with a clear majority in the Commons against no deal, and with two Tory MPs saying they are willing to vote no confidence in his government if he tries to force it through, Mr Johnson faces the very real prospect of having his plans blocked if parliament sits in the run-up to the Halloween deadline.

Ms Miller told Sky News’s Ridge on Sunday that her legal team believes prorogation in these circumstances would be “beyond the prime minister’s powers”.

“Parliamentary sovereignty is the jewel in the constitutional crown,” she said.

“To close the doors of parliament – we feel, from the case law we’ve looked at – would be beyond the prime minister’s powers. It would be an abuse of his powers to close parliament to limit the voice of the representatives that we all elect.”

The letter to Mr Johnson by lawyers Mishcon de Reya calls on him to inform Ms Miller before giving any advice to the Queen to suspend parliamentary sittings, in order that the decision can be subjected to legal challenge.

“It would seriously undermine parliamentary sovereignty for you, as prime minister, to prorogue parliament to prevent it from considering whether to legislate to prevent a no-deal Brexit,” the letter states

“You would be closing the doors of parliament to prevent it from legislating on the most important political issue of the day, when time is of the essence. In such circumstances it would be unlawful for you as prime minister to advise Her Majesty to prorogue parliament for the purpose of preventing parliament from considering the enactment of a law to stop the UK leaving the EU without a deal.”

Ms Miller said that the Supreme Court ruling in her 2017 case made clear that “parliament cannot be bypassed”.

The Supreme Court ruled in 2017 that MPs must have a voice on Article 50 (Getty)

She said: “Each of us in a democracy have the right as a citizen to exercise legitimate questions in the courts. Paying lip service to parliamentary democracy is one thing, but you actually have to actively defend it.”

She denied that her move was driven by her own opposition to Brexit and was intended to thwart the delivery of the result of the 2016 referendum.

“In the environment we are in at the moment, everything is being made into a political football,” she said. “This is not. This is very much about defending the central core of our constitution, which is that parliament is sovereign.

“This case would not change the outcome of Brexit. All it would do is give parliament the voice to shape how we exited or if we did. It would be up to parliament to decide what it did.

“The fact is, we cannot shut down parliament. It’s the central pillar of our constitution.”

Johnson supporter and former cabinet minister Priti Patel said it was “simply not acceptable” for people outside parliament to use the courts in a bid to delay or halt the Brexit process.

Ms Patel told Sky News: “Quite frankly the British public are sick to death of this.