LONDON — The speaker of Britain’s House of Commons dealt a potentially fatal blow to Prime Minister Theresa May’s ailing Brexit deal on Monday, saying the government couldn’t keep asking lawmakers to vote on the same deal they have already rejected twice.

The government intends to try a third time to get lawmakers to back the deal, ideally before May joins EU leaders Thursday at a Brussels summit where she is set to ask the bloc to postpone Britain’s departure. May has warned opponents that failure to approve the deal would mean a long, and possibly indefinite, delay to Brexit.

Speaker John Bercow scuttled May’s plan, saying centuries-old parliamentary rules prevent “the same proposition or substantially the same proposition” from being brought back repeatedly for votes in the same session of Parliament.

He said a new motion would have to be “fundamentally different. Not different in terms of wording, but different in terms of substance.”

The ruling caused an uproar on the government side of the House of Commons. Solicitor General Robert Buckland said Britain was facing a “major constitutional crisis.”

As interpreter and enforcer of Parliament’s rules, the speaker has huge power. Bercow — whose booming cries of “Orderrrrr!” have made him something of a global celebrity — has often used his office to boost the influence of backbench lawmakers, to the annoyance of the government.

“Part of the responsibility of the speaker is frankly to speak truth to power,” he said Monday.

Even before Bercow’s ruling, May faced a struggle to reverse the huge margins of defeat for the agreement in Parliament. It was rejected by 230 votes in January and by 149 votes last week.

Her goal is to win over Northern Ireland’s small, power-brokering Democratic Unionist Party. The DUP’s 10 lawmakers prop up May’s Conservative government, and their support could influence pro-Brexit Conservatives to drop their opposition to the deal.

Opposition centers on a measure designed to ensure there is no hard border between the U.K.’s Northern Ireland and EU member Ireland after Brexit.

The mechanism, known as the backstop, is a safeguard that would keep the U.K. in a customs union with the EU until a permanent new trading relationship is in place. Brexit supporters in Britain fear the backstop could be used to bind the country to EU regulations indefinitely, and the DUP fears it could lead to a weakening of the bonds between Northern Ireland and the rest of the U.K.

Talks between the government and the DUP are aimed at reassuring the party that Britain couldn’t be trapped in the backstop indefinitely.

But with no sign of a breakthrough, May’s spokesman, James Slack, said the government would only hold a vote if there is “a realistic prospect of success.”

Influential Conservative Brexiteer Jacob Rees-Mogg said he would wait to see what the DUP decided before making up his mind on whether to support May’s deal.

“No deal is better than a bad deal, but a bad deal is better than remaining in the European Union,” he told LBC radio.

After months of political deadlock, British lawmakers voted last week to seek to postpone Brexit. That will likely avert a chaotic British withdrawal on the scheduled exit date of March 29 — although the power to approve or reject a Brexit extension lies with the EU, whose leaders are fed up with British prevarication.

May has said that if her deal is approved, she will ask EU leaders this week to extend the Brexit deadline until June 30 so that Parliament has time to approve the necessary legislation.