By Guy Faulconbridge
LONDON (Reuters) - The United Kingdom's exit from the European Union is uncertain nearly three years after the 2016 Brexit vote.
Most diplomats and investors think the United Kingdom faces three main options: leaving with a divorce deal, throwing the question back to the people or exiting without a deal.
Graphic on no-deal Brexit probabilities from major banks: https://tmsnrt.rs/2UIhlyz
Following are the main scenarios:
1) BREXIT WITH A DEAL - May gets her deal approved at a third attempt and the United Kingdom leaves in an orderly fashion after a modest delay.
May's divorce treaty, the product of more than two years of negotiations with the EU, was defeated by 149 votes on March 12 and by 230 votes on Jan. 15.
She had been intending to put the deal to another vote in parliament as early as this week, but the speaker ruled on Monday that she could not do so unless the deal was re-submitted in fundamentally different form. [nL8N2153SV]
Unless May can find a way around Speaker John Bercow's ruling - such as adding an addendum or starting a new session of parliament - she will have to ask the EU to delay Brexit to avoid a no-deal exit on March 29.
Brexit Secretary Steve Barclay on Tuesday played down the possibility of cutting the parliamentary session short in order to start a new one.
Because May must now spice any deal with additional legal and procedural innovation, Bercow's ruling means she is likely to get just one more chance to put the deal to a vote.
She had warned lawmakers that unless they approved her divorce deal, Britain's exit could face a long delay which many Brexiteers fear would mean Britain may never leave.
May could discuss a delay and seek to get last-minute concessions at a March 21-22 EU summit, though with such chaos in London a crunch decision on Brexit might be delayed until the following week.[nL8N2154G1]
The EU has repeatedly said the Withdrawal Agreement is the only deal on the table and May's spokesman said Britain would not be seeking to renegotiate the most contentious part - the Irish border plan.
If May is looking for a legal fix, though, she could seek a change to the accompanying Political Declaration.
Sources in Brussels said on Monday that Britain could ask for a Brexit delay even after the summit, suggesting that the decisive moment for Brexit might still be some days ahead.
One possible way out for May would be a Brexit delay until the end of 2019, with an option to leave earlier should her deal get passed. Ultimately, May might have to offer a date for her own resignation to win enough Conservative votes for her deal.
To get her deal through parliament, May must win over at least 75 lawmakers: dozens of rebels in her own Conservative Party, some Labour lawmakers, and the Northern Irish Democratic Unionist Party (DUP), which props up her minority government.
Jacob Rees-Mogg, chairman of the European Research Group of eurosceptics in Britain's House of Commons, signaled he could fall in behind the deal. [nL8N2152DJ]
Many banks and investors still say her deal could be struck and approved, and cite previous EU crises such as the Greek debt crisis, where solutions were found at the eleventh hour.
"I think MPs (lawmakers) will see sense and approve the Meaningful Vote before March 29," said Matthew Elliott, the head of the 2016 campaign for leaving the European Union, told Reuters after Bercow's ruling.
"The most likely outcome at this juncture is the deal going through," Elliott said. "When it becomes apparent that the only extension on offer from the EU is long, tortuous and with lots of conditions, I suspect enough MPs will get behind the deal for it to pass."
If May's deal fails, or if another vote on the same deal is prevented, another option is that parliament at some point takes control of Brexit and lawmakers seek a closer relationship with the EU, staying in the EU customs union.
Lawmakers could seek indicative votes on a way forward and there might be a majority for a softer Brexit than May's deal. To avoid that, May could call a snap election, though her party does not want one.
Another option, being pushed by some lawmakers is a referendum on May's Brexit deal, though such a vote, were it ever called, would effectively become a referendum on EU membership.
2) BREXIT REFERENDUM - May's deal fails and a long delay allows the campaign for another referendum to gain momentum.
It is far from clear how the United Kingdom would vote if given another chance.
An often chaotic set of votes in parliament last week has shown that none of the alternatives to May's deal - such as leaving with no deal, a referendum or allowing parliament to decide how to leave - can muster a majority among lawmakers yet.
In the June 23, 2016 referendum, 17.4 million voters, or 51.9 percent, backed leaving the EU while 16.1 million, or 48.1 percent, backed staying.
While many surveys ahead of the vote incorrectly predicted that the United Kingdom would vote to stay in the club it joined in 1973, polls now suggest no great desire for a second referendum and indicate that many voters, fatigued by the political squabbling, would be happy to leave without a deal.
Corbyn, who voted against membership in 1975 and gave only reluctant backing to the 2016 campaign to remain in the EU, has given ambiguous backing for another referendum, saying he would push for one alongside a national election.
When asked if he would vote to remain in the EU in a possible future referendum, Corbyn said on Sunday: "It depends what the choice is in front of us."
At the highest levels of government, there are worries that a second referendum would exacerbate the deep divisions exposed by the 2016 referendum, alienate millions of pro-Brexit voters and stoke support for the far-right.
Already, many supporters of Brexit, and even some lawmakers, say the elite has sabotaged the EU divorce and is trying to subvert the will of the people.
It is far from clear how the United Kingdom would vote and even if it did vote to remain, Brexit supporters might demand a third and decisive vote.
A new party backed by Nigel Farage, the insurgent who helped shove Britain towards the EU exit, has a message for the country's leaders: The foundations of the political system will explode if Brexit is betrayed.
3) NO-DEAL EXIT - The chaos in London is such that parliament cannot find a way to approve May's deal or find another divorce deal option, and after one or more delays, the EU says it will extend no longer. The United Kingdom then leaves without a deal.
Lawmakers on Wednesday voted 321 to 278 in favor of a motion that ruled out a potentially disorderly "no-deal" Brexit under any circumstances.
While the approved motion has no legal force and ultimately may not prevent a no-deal exit, it carries considerable political force.
Still, as the March 29 exit date is set in law, the default is to leave on that date unless May agrees a delay or parliament changes the law.
"You either have a deal, you have no deal, or you have no Brexit," said Brexit Secretary Steve Barclay.
While an extension would avoid a no-deal exit on March 29, the potential for a no-deal Brexit would remain if the British parliament was unable to approve a deal.
And the European Union's 27 other members must unanimously approve a delay to Brexit.
Barclay has said Britain should not be afraid of leaving without a deal if it cannot get a divorce deal approved.
No-deal means there would be no transition so the exit would be abrupt, the nightmare scenario for international businesses and the dream of hard Brexiteers who want a decisive split.
Britain is a member of the World Trade Organization so tariffs and other terms governing its trade with the EU would be set under WTO rules.
(Editing by Anna Willard and Giles Elgood)