Sending the Queen to the EU to request an extension to Brexit and prevent a no-deal being forced through by the next prime minister is a "ridiculous idea", a former Supreme Court justice has said.
Lord Sumption criticised the notion after it was reported that senior Tories have touted using a parliamentary device known as a humble address to get the monarch to act.
The suggestion is an example of the extraordinary resistance front-runner Boris Johnson may face if he attempts to force through a deal-less departure against the will of MPs.
Newsnight reported that the Queen could be asked to exercise her right as head of state to travel to the next EU summit and seek a delay to Brexit if the next PM ignored a vote rejecting no-deal.
Lord Sumption told BBC Radio 4's Today programme: "I agree, it's a ridiculous idea.
"I think, first of all, that MPs putting forward a particular point of view should pipe down on the question of bringing the Queen in because that very fact puts her in an awkward position and makes it more difficult for her and reduces her options."
He also called for the Queen to have a source of advice independent of the Government on the limits of ministers' powers.
Lord Sumption, who has retired from the top court, said the constitutional basis on which the Queen must listen to the Privy Council is that the Government commands a majority in the Commons, which it does not.
"So, using the powers of the crown to defeat the majority is a very strange idea. But we're in an entirely new territory," he said.
The reported manoeuvrings against a potential Johnson premiership are the latest apparent attempts to thwart his authority.
He has refused to rule out suspending Parliament in order to ram home a no-deal exit unlike the Tory leadership race underdog, Foreign Secretary Jeremy Hunt.
But the scale of the task was exposed in the Commons on Thursday when Cabinet ministers expecting to return to the backbenches under his leadership sent a firm signal.
MPs voted by a majority of 41 to back a measure that could prevent Mr Johnson suspending Parliament in order to force through a no-deal Brexit, with 17 Tories rebelling.
Crucially, Chancellor Philip Hammond, Justice Secretary David Gauke, Business Secretary Greg Clark and International Development Secretary Rory Stewart did not take part in the vote.
The quartet could be a thorn in Mr Johnson's side as he struggles with an administration propped up by the DUP.
The majority could be reduced to just three if the Conservatives fail to hold Brecon and Radnorshire in the August 1 by-election.