Rejecting the Brexit deal will get Britain nowhere, EU leaders warned Sunday.

"This is the only deal possible," Commission President Jean-Claude Juncker declared, issuing a pointed message to the British House of Commons shortly after EU27 leaders approved the deal agreed by Theresa May.

At present, the British prime minister does not have the votes needed for ratification — a grim reality that she did not disguise from her EU colleagues on Sunday. Indeed, a clear majority seems to exist only in opposition to a potentially catastrophic cliff-edge scenario. But Juncker and other EU leaders sought to insist that there would be no renegotiating the divorce treaty.

"I am totally convinced that this is the only deal possible," Juncker said, standing with Council President Donald Tusk and the EU's chief negotiator, Michel Barnier, at a news conference following an extraordinary EU leaders' summit. "Those who do think by rejecting the deal that they would have a better deal will be disappointed in the first seconds after the rejection of this deal."

“We managed to achieve a diplomatic masterpiece” — Angela Merkel

Other EU leaders were equally adamant. “There isn’t a plan B," Irish Prime Minister Leo Varadkar declared. "The truth is what we have here is the best deal that’s available both for the United Kingdom and for the European Union.”

May, too, sought to reiterate the point. After months and months of countering the rhetoric of EU leaders, she now finds herself echoing them, more aligned (on this at least) with Juncker than with say, Boris Johnson, and dozens of other MPs in her own party.

"The point Jean-Claude Juncker was making, and it’s been reiterated by others, was that if people think somehow there’s another negotiation to be done, that’s not the case,” she told reporters after the summit. “This is … the result of what [have] been tough and difficult negotiations over a significant period of time. We’ve said this is the deal that’s on the table, this is the best possible deal. It’s the only possible deal.”

May’s fight for her deal at home is also a fight for political survival. She batted away two questions on whether she would resign ("this isn't about me") and one about her Foreign Secretary Jeremy Hunt’s candid refusal to rule out, on the BBC’s Andrew Marr Show on Sunday morning, that the government could collapse.

“It’s not possible to rule out anything,” he said.

While the EU's posture is clearly aimed at helping May build the case that the choice before parliament is her deal or no deal, British MPs are well aware that there are any number of alternatives and potential escape hatches, including a possible extension of the negotiating period beyond the March 29, 2019 deadline. And it is far from clear that EU27 leaders would be prepared to allow a no-deal scenario — with highly uncertain economic and legal consequences, not only for the U.K. but for their own countries as well.

Jeremy Corbyn, the leader of the U.K.'s opposition Labour Party, was quick out the blocks to remind May that parliament does not consider this a done deal. “We will work with others to block a no-deal outcome, and ensure that Labour’s alternative plan … is on the table,” he said.

Tusk, for his part, seemed to leave just the slightest bit of wiggle room. "We shouldn't, I think, speculate on what if," Tusk said at the closing news conference. "For sure I am not a fortune teller. This is why it is, for me, too difficult to predict what will be the result of the ratification process."

Sporting a funereal black tie, French President Emmanuel Macron confessed a "certain sadness to see Europe shrinking." But he too sought to avoid the question of what would follow a rejection of the deal by MPs. “[It is] not up to me to speculate on the British vote,” he said, adding that he would "not interfere."

The endorsement by the European Council of the 585-page Withdrawal Agreement and an accompanying Political Declaration laying out the framework of a future relationship, came a month later than officials had initially hoped. But the endorsement of the EU27 is nonetheless well in time to complete ratification and assure the U.K.'s orderly withdrawal after more than four decades of membership in the European club.

But in the end, EU leaders found themselves confronting the same divisions, political turmoil and vexing uncertainty that have gripped the U.K. since before the June 2016 referendum. And Sunday's extraordinary summit showed just how upside-down and inside-out Brexit has turned European politics. EU leaders helped dutifully to effectuate a policy that they firmly believe is neither in the EU's nor the U.K.'s interests, while May defended her agreement by trashing much of what EU leaders hold dear.

In a telling moment during her press conference, May was asked by a German journalist whether she shares Chancellor Angela Merkel’s professed “sadness” at the U.K.’s departure.

“Well, no,” came the somewhat brutal answer, prompting awkward laughter in the room, before May recovered: “But I recognize that others do.”

If she’s going to sell this deal, May can’t be sad about it. Indeed, she pledged to argue for it “with all my heart” before a parliamentary vote that she confirmed would take place “before Christmas.”

The prevailing mood from EU leaders was one of somber regret, however. "A country leaving the EU doesn't give rise to the raising of Champagne glasses or applause," Juncker said, adding later: "This is not a glorious moment for the European integration and for the pursuit of what we have started in common so many years ago."

Dutch Prime Minister Mark Rutte said there was "no applause, no flowers."

Barnier, who led the effort to pull together the deal, confessed after the news conference that the work had taken its toll. "I am a little tired," he said.

The huge uncertainty still looming over the process in many ways obscured the historic nature of the day, in which 27 EU leaders unanimously endorsed the first-ever departure of a member of the European Union.

Merkel nodded to the extraordinary achievement of reaching a deal. "We managed to achieve a diplomatic masterpiece," Merkel said at her own news conference. But she also made clear that there was little for leaders to do other than put their stamp on an agreement that none of them wish was actually necessary. "I wish you all a nice Sunday," she said. "Our contribution was that we weren't late today."

"I don’t think that Britain will be a third country like other third countries are third countries" — Jean-Claude Juncker

Merkel, Juncker and others said they intend to steer clear of the proceedings in the U.K. parliament.

Asked if she would help May win ratification, Merkel said: "[One should do] nothing that one's not asked to do."

Juncker was pressed on whether British MPs should keep in mind the EU's view that the best outcome would be no Brexit at all.

"I don't think it would be a good idea for the three of us to lecture the House [of Commons], because the house is the house and members of parliament are members of parliament and they have to make up their minds," Juncker said.

He did, however, offer a reminder that the divorce treaty was always supposed to be the easiest part of the process and that the truly difficult work — negotiating the specific terms of a future relationship — still lies ahead.

"By opposition to what some people think, the work is not over," he said. "The biggest part of the work which has to be done, will start now. Because divorce is a tragic moment, as some of you must know. Payments have to be made. But the future understanding is one which has to be constructed. I don't think that Britain will be a third country like other third countries are third countries. There is between us something which are the remainings of love."

Eddy Wax, Jacopo Barigazzi, Florian Eder, Maïa de La Baume and Eline Schaart contributed reporting.