BRUSSELS (REUTERS) - European Union leaders warned the British parliament not to wreck Theresa May's Brexit deal, describing a package agreed with the prime minister at a summit on Sunday (Nov 25) as the best that Britain will get.

"Those who think that, by rejecting the deal, they would get a better deal, will be disappointed," European Commission President Jean-Claude Juncker told reporters after the 27 other EU leaders formally endorsed a treaty setting terms for British withdrawal in March and an outline of a future EU-UK trade pact.

Asked whether there was any chance Brussels would reopen the pact if an alliance of pro- and anti-Brexit forces votes it down in the House of Commons, Juncker simply stressed "this is the best deal possible" - although summit chair Donald Tusk sounded more guarded, saying he did not want to consider hypotheticals.

May used a post-summit news conference to make a sales pitch for the deal, telling television viewers at home that it was the "only possible deal", offering control of UK borders and budgets while maintaining close cooperation with EU regulations that was good for business and the security of the broader region.

"In any negotiation, you do not get everything you want," May said. "I think the British people understand that." Parliament's vote could open the door to a "brighter future" or condemn the country to more division. "I will make the case for this deal with all my heart," she concluded, declining to answer whether she would resign if parliament rejects it.

German Chancellor Angela Merkel, the bloc's veteran guiding force, echoed that unwillingness to speculate on what she called a "historic day" that was both "tragic and sad". But Austrian Chancellor Sebastian Kurz, whose country chairs EU meetings till the end of the year, said there could be no more negotiations.

"There is no Plan B," said Dutch Prime Minister Mark Rutte."If anyone thinks in the United Kingdom that by voting No something better would come out of it, they are wrong."

The only Plan B was preparing a possible no-deal scenario in which Britain crashes out on March 29 into legal limbo, roiling Europe's economy, a senior EU official said.

In May's exchanges at the summit, there was no discussion of what may happen if parliament rejects the deal in a vote likely to take place just before the next EU summit on Dec 13 to 14.

NO CHAMPAGNE

Amid praise for Michel Barnier and the Commission's team of negotiators for bringing home a deal after 18 months of gruelling talks, Juncker said it was "no time for champagne", as one of Europe's great powers walks out after its 2016 referendum. The harder work of building new relations now lies ahead, he added.

The 27 leaders took barely half an hour at a summit in Brussels to rubber-stamp the 600-page withdrawal treaty, aimed at an orderly exit on March 29 to be followed by two to three years of a status-quo transition period. The outline of a future trading and security partnership was just 26 pages long. May, who joined them shortly afterwards for an hour, now faces a struggle to get the deal a fractured parliament in which her own divided Conservative party lacks a majority.

British Foreign Secretary Jeremy Hunt said that the Brexit deal was a "staging post" towards Britain getting everything it wanted from leaving the EU, but that the arithmetic for getting the deal approved was looking "challenging".

European Council President Tusk said the bloc was determined to have as close as possible a partnership with Britain, which has long been sceptical about EU integration: "We will remain friends until the end of days. And one day longer," he said.

French President Emmanuel Macron said Britain's Brexit vote showed Europe needed reform. He stressed that Paris would hold Britain to tight EU regulations, in return for giving it easy trade access.

He also foreshadowed coming, fraught, negotiations by demanding access to British fishing grounds after Brexit. Lithuanian President Dalia Grybauskaite said there were at least four possible outcomes if parliament blocks the package. She named three - that Britons would hold a second referendum, hold a new election to replace May or return to Brussels to try and renegotiate the package. A fourth is that Britain will simply crash out.

Both sides have been making preparations for such a "no deal" scenario, though the EU insists Britain has more to lose. The pound has strengthened since the deal came together over the past 10 days, but companies and investors remain nervous.

IRISH QUESTION

Northern Ireland's Democratic Unionist Party, which props up May's minority government, has said it would try to block the deal because it binds London to many EU rules and the DUP fears it could weaken the province's ties to Britain.

DUP leader Arlene Foster said on Sunday she would "review"the agreement to back May's Conservative government if the Brexit divorce is passed by Britain's parliament.

Sunday newspapers said different factions in her own Conservative party were preparing alternative plans to keep Britain closer to the EU should May's deal fail. Wrangling over how to keep open troubled Northern Ireland's land border with the EU without creating barriers with the Irish Republic dogged much of the Brexit talks.

Britain's 300-year-old naval base in Gibraltar on Spain's southern coast, had also threatened to derail plans.