Britain Will Have to Pass Theresa May’s Brexit Plan, E.U. Warns

BRUSSELS — If Britain ever wants an orderly exit from the European Union, it must pass the withdrawal agreement painfully negotiated with the bloc, Michel Barnier, its chief Brexit negotiator, warned on Tuesday.

To date, the British Parliament has already rejected the agreement three times by margins varying from historic to hefty, though Prime Minister Theresa May is preparing for another shot, perhaps on Friday.

Even if Britain asks for a long extension to Article 50, Mr. Barnier said, the withdrawal agreement will not be reopened, and negotiations on the country’s future relationship with the European Union cannot legally take place until Britain is no longer a member.

Having failed to achieve a breakthrough on Monday, a group of lawmakers working across party lines were on Tuesday planning a new strategy to try to avert the increasing risk of a no-deal Brexit.

If approved in a vote expected to take place Thursday, the measure introduced by a Labour member, Yvette Cooper, would require Mrs. May to request another delay.

But the proposed legislation, which will mostly likely be discussed on Wednesday, does not specify what the extra time would achieve, so it remains unclear whether such a request would satisfy European Union leaders. They have said that Britain could get another extension only if it has a new strategy, such as holding a general election or second referendum, and if it agrees to take part in the European elections in May.

The cabinet held two meetings on Tuesday as Mrs. May attempted to find a way out of the crisis. Though she said she wanted to avoid leaving without a deal and noted that Parliament would try to stop it, the prime minister’s office also said that a no-deal exit was the default scenario if there were no agreement.

Mrs. May’s instincts are probably to try to use the twin threats of a long extension and no deal to try to force her deal through. But it remains unclear whether the speaker of the House of Commons, John Bercow, will permit another vote on a plan that has been rejected, in different forms, on three occasions.

Britain could also leave without an agreement, which Mr. Barnier said is “day after day more likely.” He insisted that the European Union was prepared for this scenario.

But even then, he said, Britain would have to return to talks with Brussels in a badly strained relationship to manage day-to-day dealings, including air transportation, trade and customs. The bloc could then negotiate a free-trade agreement with Britain, as with Japan or Canada, but key issues would “still be on the table,” Mr. Barnier said, listing protections for citizens’ rights, preventing a hard border between Northern Ireland and Ireland, and Britain’s legal and financial obligations to Brussels.

It would be much simpler for Parliament, which Monday night could find no majority for the Brexit plan, to pass the legally binding withdrawal agreement, Mr. Barnier said. Then the two sides could immediately begin to negotiate their future relationship, since the nonbinding political declaration “leaves the door open for a variety of outcomes” and could be easily adjusted to Britain’s wishes.

“If the U.K. still wants to leave the E.U. in an orderly manner, this agreement, this treaty, is and will be the only one,” Mr. Barnier said. “If there is no deal, there is no transition.”

Mr. Barnier spoke and took questions on Tuesday at the European Policy Center, a think tank based in Brussels.

The mood in Brussels has been somber as the British Parliament floundered in its attempts either to pass the withdrawal agreement or to find a majority for any alternative. But that agreement, including the so-called Irish backstop, will not change, Mr. Barnier and other European leaders have said.

Some member countries, like France, tend to favor a no-deal Brexit so the issue does not affect European parliamentary elections at the end of May.

The fear is that Brexit will be taken up by Europe’s various populist and euroskeptic parties as another sign of the supposed indifference of Brussels to democratic expression.

But others, like Germany, believe that Britain is a crucial neighbor and partner in trade and defense, and argue that it should be given a longer extension. The heads of state and government are to meet in Brussels in an emergency session on April 10, two days before the current Brexit deadline, to hear what Britain has decided.

To get an extension, Britain must tell the other member states why it wants one — for a general election, another referendum or some other reason that promises a way out of the impasse rather than just allowing more time for bickering and deadlock.

But a long extension has political and economic costs for both sides, Mr. Barnier noted, citing uncertainty that is damaging business relationships and investment and the need for the European Union to concentrate on other pressing issues, like migration, economic growth, inequality, refugees and terrorism. The leaders, he said, would have to “balance the different costs, between no deal, even if by accident, and a long prolongation.”

The European Union will protect its single market no matter what, Mr. Barnier said, since 500 million people and 22 million businesses depend on it. “We will never fragilize or unravel the single market,” he said in English. “If we fragilize or unravel the single market, we are lost.”