A lawsuit would “ask the courts to determine who, Parliament or the prime minister, gets to decide whether we no-deal,” Maugham said. “For me, the answer should obviously be Parliament.”

“It’s not theoretical,” Miller said about her plans to sue. “We have already started the protocol process — if he should go there.”

Johnson, the favourite to succeed Prime Minister Theresa May when she steps down next week, has refused to rule out suspending Parliament to get Brexit done by Oct. 31. Miller’s comments follow a significant hardening of position by both Johnson and his rival for the top job, Foreign Secretary Jeremy Hunt. They both say they won’t accept any form of Irish backstop in a deal with the European Union — a key provision designed to keep the border open after Brexit and a red line for the bloc in negotiations.

Former Prime Minister John Major has also said he would be ready to take the government to court if the incoming leader tries to suspend Parliament, a position Chancellor of the Exchequer Philip Hammond has also backed.

But Miller, who is a founding partner at SCM Direct, ruled out joining forces with anyone if it meant “politicizing” the issue. “It’s got to be about the black-and-white letter of the law,” she said.

The power to suspend Parliament lies with the monarch at the request of the prime minister, which Miller said could leave Queen Elizabeth II in a “very, very difficult position.” Declining the premier’s request would make the monarch an “active participant in the political scene.”

Miller also said she sees the chances of a second referendum on Brexit as “almost zero” due to the divisions in the main opposition Labour Party on the issue. Parliament has so far shown there’s no majority for another vote.

As early as 2016, Maugham helped raise money through crowdfunding for the first public step in Miller’s landmark case, where she used the courts to force the government to get parliamentary approval before beginning Brexit talks.

Two years later, Maugham brought his own Brexit case in Scotland, with the ultimately successful goal of getting the issue in front of the EU Court of Justice. There, the EU’s top judges in Luxembourg ruled that the U.K. could unilaterally revoke its so-called Article 50 notice at any time before it actually left the bloc.

But his fresh challenge wouldn’t be connected to Miller’s.

There are “different views about the best way to run litigation, and the issue is sufficiently important” for the separate cases to be heard, Maugham said. He may again bring his case in Scotland and it would likely be crowdfunded, he said.