SHARM EL-SHEIKH, Egypt >> Britain’s main opposition party took a big step today toward backing a second Brexit referendum amid rising concerns the country could crash out of the European Union next month without a deal.

The EU also warned that Britain faced the prospect of either a chaotic exit from the bloc or delaying its planned departure date. European Council President Donald Tusk, who chairs the summit meetings of EU nation leaders, said it would be “rational” for the March 29 Brexit date to be extended.

In a move that could change the Brexit debate in Britain, the Labour Party said it would back a second public vote if the House of Commons reject its alternative withdrawal plan later this week — as is widely expected.

Leader Jeremy Corbyn told Labour lawmakers the party “will do everything in our power” to prevent Britain leaving the EU without a deal on the withdrawal terms and future relations with the remaining 27 members.

But Corbyn also said Labour would oppose any agreement based on the “overwhelmingly rejected” one negotiated by Prime Minister Theresa May’s government. Parliament has refused to approve the agreement since the government and the EU reached it late last year.

As a result, Labour plans to put forward or support an amendment to the deal on holding another public vote “to prevent a damaging Tory Brexit being forced on the country,” he said.

Voters narrowly approved the June 2016 referendum to pull Britain out of the EU. Labour previously said it would support a referendum as a last resort if it could not secure a new general election or make changes to May’s divorce deal.

Without support from the Labour Party, there is little chance of a second referendum taking place.

The move is likely to cheer many Labour Party members, who have backed calls for a so-called “people’s vote.” The party leadership has been reluctant to change tack, not least because majorities in many of the districts Labour lawmakers represent, particularly in northern England, backed Brexit in the referendum.

Though Labour’s move is potentially a game-changer, the path to another referendum is far from clear. Another Brexit vote would require the support of numerous lawmakers from the governing Conservative Party, for example.

After lawmakers rejected May’s deal last month, the prime minister has sought to get changes from the EU over a provision for the border between the U.K.’s Northern Ireland and EU member Ireland. The mechanism, known as the backstop, is a safeguard that would keep the U.K. in a customs union with the EU in order to remove the need for checks along the Irish border until a permanent new trading relationship is in place.

May wants to change the deal to reassure British lawmakers that the backstop would only apply temporarily.

But EU leaders insist that the legally binding Brexit withdrawal agreement, which took a year and a half to negotiate, can’t be reopened.

The impasse has raised concerns that Britain will leave the EU without a deal, a scenario that would likely mean new tariffs on British exports and serious disruption to trade between the two sides. The Bank of England has warned that the British economy could shrink by 8 percent in the months after a disorderly Brexit.

May has said a new vote on any revised Brexit deal won’t be held this week and could come as late as March 12. A number of British lawmakers are seeking to wrest control of the process away from government and are looking to get support for an amendment that would require May to seek an extension to the Brexit date if Parliament fails to back her deal.

Labour lawmaker Yvette Cooper, one of those behind the move, said it was irresponsible of the government that just a few weeks before Brexit “we still don’t know what kind of Brexit we are going to have and we’re not even going to have a vote on it until two weeks before that final deadline.”

“I don’t see how businesses can plan, I don’t see how public services can plan and I think it’s just deeply damaging,” Cooper told the BBC.

The EU’s Tusk warned that the chances of a withdrawal agreement being concluded in time are receding, and that sticking by the planned Brexit date would be too risky.

“I believe that in the situation we are in, an extension would be a rational solution,” Tusk told reporters at an EU-Arab League summit in Egypt after talks with May that he said included discussions over extending the Brexit process.

He said all 27 member states “will show maximum understanding and goodwill” to make possible such a postponement — a decision that would require a unanimous vote from them.

However, May insisted she could deliver on the set date.

“It is within our grasp to leave with a deal on 29th of March and I think that that is where all of our energies should be focused,” May said.

She said that “any delay is a delay. It doesn’t address the issue. It doesn’t resolve the issue.”

Dutch Prime Minister Mark Rutte warned her against “sleepwalking” into a chaotic Brexit next month and that it was time for Britain to step up and clinch a deal.

“It’s absolutely unacceptable. And I think your best friends have to warn you for that,” Rutte told the BBC. “Wake up. This is real.”