By Gabriela Baczynska, Jan Strupczewski and Alastair Macdonald
BRUSSELS (Reuters) - EU negotiators cracked open champagne after watching Theresa May agree to their Brexit deal on television on Wednesday night but even then they knew the "orderly withdrawal" it foresees for Britain is far from certain.
"We're celebrating now but we'll be in tears when the UK parliament votes it down," one official said, hours before the British prime minister's own Brexit secretary and a handful of other ministers resigned and she faced talk of being ousted.
As the other 27 EU national leaders prepare for a summit with May a week on Sunday, Nov. 25, to cement the deal setting out legal terms for Britain's departure from the bloc on March 29, the political meltdown in London has left officials asking what will happen next -- and coming up with few clear answers.
The array of possible developments in Britain -- from May succeeding in her plan to squeak the deal past lawmakers to her being deposed or Britons deciding to hold a new referendum on Brexit -- leaves an even more complex array of potential responses from Brussels and continental capitals.
"Crazy. Crazy. It's really hard to get your head around it," said one senior EU diplomat. "I just can't say what we'd do."
Senior officials said the EU executive will have secret outline plans for all eventualities, which might even include Britain changing its mind and asking to stay. But the most intense planning now is for it crashing out without a deal.
"NO ROOM FOR MANOEUVRE"
"The EU is prepared for a final deal with the United Kingdom in November," said Donald Tusk, who will chair next Sunday's summit. "We're also prepared for a no-deal scenario. But of course we are best prepared for a no-Brexit scenario."
That latter remark, a senior official stressed, was meant partly in jest, although senior European parliamentarians have also spoken in support of Britons seeking a second referendum.
One immediate possibility is that Britain comes back to seek amendments to the draft, which has not been signed off on by EU governments, and/or concessions in an outline accord on future trade and security relations also being worked on for Nov. 25.
EU diplomats said that, as leaders including German Chancellor Angela Merkel and May herself have said, there does not seem much scope on either side for movement.
"We think we have, on both sides, exhausted our margin of manoeuvre under our respective mandates," one senior EU official said. "If someone wishes for changes they also have to take responsibility and assess what this does to the process."
Another did not rule out new talks running into next month: "Even without changing the deadlines you could still try to reach a different negotiated outcome," he said. "A lot would depend on the signals we would get from London, it is difficult to say at the current juncture -- there is so much chaos there."
"SICK AND TIRED OF THIS"
Leaders seem willing in principle to give Britain some more time to settle its position, extending the deadline for leaving beyond March 29 - a decision that requires unanimity. But any extension would depend on there being a reasonable expectation that a new deal was close - and not exceed a few weeks.
Europeans vote for a new parliament on May 23-26. Assuming Britons will not vote, they will need to be out of the EU by then to avoid legal questions over the legislature's legitimacy.
Allowing Britain to hold a second Brexit referendum would therefore have very significant time constraints. It would also raise questions on whether and how Britain could opt to stay - a question EU judges are set to address on Nov. 27.
While British opponents of Brexit hope that there could be a way to stay, and Tusk and others insist they would be welcome, the prospect of a toxically divided major nation prolonging its love-hate relationship with the bloc in another narrow vote is not one that generates any enthusiasm on the continent.
"People are sick and tired of this," said one Brussels insider, noting EU preparations for coping with a rocky "no deal" in March. "Brexit is a real drag on EU business. There is a real desire to get it over with -- one way or another."
(Writing by Alastair Macdonald; Editing by Peter Graff)