Campaigner Gina Miller is planning to launch urgent legal action to prevent Boris Johnson from crashing Britain out of the EU.
The businesswoman has assembled a legal team to prevent the Tory leadership frontrunner from shutting down parliament in order to drive through a no-deal Brexit without the support of MPs.
The team from law firm Mishcon de Reya have written to Mr Johnson saying that such a move would be "constitutionally unacceptable" and unlawful, according to The Observer.
Any attempts to prorogue parliament would lead to an urgent challenge in the courts, they said.
Ms Miller's team include QCs Lord David Pannick and Tom Hickman, who successfully forced Theresa May to grant MPs a vote before triggering article 50 in 2017.
In a letter seen by the newspaper, they make it clear to Mr Johnson that shutting down parliament would involve the use of Royal prerogative powers, which the courts would seek to limit.
"Such powers cannot be exercised in a manner which is contrary to fundamental constitutional principles," they wrote.
"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 added.
Ms Miller said the letter sent on Thursday "was to say that if he became prime minister that we believe that that would be beyond his powers, and also relying on the judgment in my case in 2017 where the Supreme Court expressly said that Parliament could not be bypassed".
Speaking about the prospect of prorogation of Parliament on Sky News's Sophy Ridge On Sunday, she said: "We think that it's beyond the Prime Minister's powers because Parliamentary sovereignty is actually the jewel in the constitutional crown and to bypass and to close the doors of parliament, we feel from the advice and the case law we've looked at, that that would be beyond a prime minister's powers.
"It would be an abuse of his powers to close Parliament, to get through or to not get through, to limit the voice of the representatives that we all elect."
It comes after Mr Johnson has continuously refused to rule out shutting down parliament throughout his leadership campaign as the October Brexit deadline looms.
The Tory favourite for replacing Mrs May has said the UK must leave the EU on October 31 "deal or no deal".
His opponent Jeremy Hunt, however, has ruled out prorogation of parliament to prevent MPs blocking no deal.
Ms Miller told the Observer that she and her team were "absolutely ready to go" to prevent what she called an outrageous attempt to bypass parliament.
Her latest move will add to the Brexit furore as the country faces political and economic uncertainty ahead of the October 31 deadline.
Sir John Major, the former Tory prime minister said last week that he would also be prepared to go to court to stop Mr Johnson proroguing parliament.
“I served in parliament for over 20 years. I’m very proud to have done so,” said Major. “I have huge admiration for our parliamentary traditions.
“I’m not going to stand by and see them disregarded in this fashion. It is utterly, utterly and completely the wrong way to proceed."