LONDON — A specter is hanging over Westminster — the specter of “no deal” killing off Brexit altogether.

Even as divorce negotiations stall in Brussels, very few in London want to walk away from the European Union without a deal.

The nightmare now being discussed privately by many MPs is that Prime Minister Theresa May is too weak either to push through the concessions necessary to get a deal with Brussels — most obviously in the form of a cash payment to the EU27 — or to survive the consequences of failure to reach an agreement.

In the event of such an impasse, Britain either falls off the cliff at the end of the Article 50 period, finds a way to delay Brexit to get more time to find a way through the crisis, or reverses the decision to leave altogether.

“It would be a very acute political crisis,” said one Conservative MP not prone to hyperbole.

Such calculations ratchet up the pressure on the prime minister — from Brexiteers fearful that failure to come to an agreement could trigger a political crisis that ends with Britain staying in the EU, at least in the short term, and from Remain supporters determined to do everything in their power to stop Britain crashing out of the EU.

British negotiators fear that without decisive leadership, 27 red lines could emerge, leaving such limited room for maneuver.

One senior Conservative MP, a former Cabinet minister, said no deal was akin to committing harakiri — to avoid the shame of EU capture — and did not rule out voting to bring down the government rather than impose such an outcome on the country, even if that meant ushering in Prime Minister Jeremy Corbyn.

Many in Westminster now fear "no deal" is the most likely scenario. "I think we are heading for no deal," Shadow Foreign Secretary Emily Thornberry told the BBC's Andrew Marr Sunday. "I think that that is a serious threat to Britain and it is not in Britain's interest for that to happen and we will stop it."

A leading Corbynista put it more bluntly: “[Brexit] won’t happen because the government are a useless bunch of spivs and they’re f--king it up.”

Yet as talk of a no deal and speculation about how such a scenario would play out gets louder, some MPs — on both sides of the Brexit debate — now foresee the phoenix of Remain rising from the ashes of a failed negotiation.

Here are five scenarios in which a failure to agree a Brexit deal with Brussels could stall Britain's exit from the EU.

1. No deal threat to Theresa May

Despite the warm words of German Chancellor Angela Merkel at the European Council Thursday, British negotiators are far from certain that a compromise deal on the exit fee can be found.

The European Commission is not looking for a number, but the problem remains — there is no equation that has been found to calculate the bill that would be acceptable to the EU that is not immediately toxic for London. To break the deadlock, big EU member countries have begun to engage their own finance departments, one senior U.K. official said.

Should an agreement remain elusive at the next European Council in December, the markets could begin to take fright and Euroskeptic calls for the talks to be abandoned will grow.

Early failure in the negotiations could be fatal for May and the Tories’ grip on power. Calls within the party for a hard-liner to take over with a pledge to leave without a deal unless the EU backs down would inevitably grow. But to win a majority in the Commons for such a stance — and a mandate from the country — a new election might be necessary. This would be a moment of maximum danger for the Conservatives and for Brexit, with Labour almost certain to oppose.

The prospect exposes the precariousness of May's negotiating position. To face the indignity of a formal leadership challenge, just 48 Conservative MPs need to sign a letter of no confidence in her leadership and submit it to the chairman of the backbench 1922 committee, Graham Brady. However, if this were to happen and a hard-line Brexiteer take her place as prime minister, the Labour Party could move a vote of no confidence in the new government to test whether centrist Conservative MPs are prepared to support the new "no deal" policy.

Should the government lose the vote — which it could if only the DUP broke ranks at the prospect of a hard border being imposed with the Republic of Ireland — a new election would be called within two weeks, assuming Corbyn's Labour was unable to form a government itself.

While Corbyn is so far committed to Brexit, his party remains ambivalent at best. A lengthy "holding pattern" Brexit or a second referendum could open up, Conservative MPs fear.

2. EU disunity

Much is made of the European unity over Brexit — indeed, even British negotiators privately praise the discipline shown by the 27.

But U.K. officials fear — yes, fear — that this could break down if the talks move onto the future trade relationship, according to conversations with leading negotiators and Cabinet ministers.

If the U.K. had limitless time, a policy of picking one country off against another might be desirable, but with a ticking clock imposed by Article 50, no such luxury is available. Britain wants a deal and needs the EU to decide swiftly what its preferred relationship with Britain should be after Brexit.

After a year of fractious internal debate in Britain, something approaching consensus has emerged around a souped-up Canada-style deal, creating a European “free-trade area,” with a customs agreement. Red lines have been drawn on free movement (against) and free trade with other countries (for), but much else is open to negotiation.

Which sectors of the economy, though, does Europe want included in a free-trade deal? Financial services? Some countries do, some don’t. Fishing? Norway has an opt out. Goods? Surely.

British negotiators fear that without decisive leadership — not a hallmark often associated with Brussels — 27 red lines could emerge, leaving such limited room for maneuver, it would be impossible for the U.K. government to justify handing over such huge amounts of money.

“The PM has to be able to sell the deal she’s got — she’ll need to buy off the arch Brexiteers,” said one Tory minister.

Facing a choice between trying to delay the exit date or leaving without an agreement — or, at best, a limited and expensive "bad deal" — the government may be forced to call an election, throwing everything back into the air. As before, May could lay a motion in the House of Commons setting the date for an election, which would require two thirds of MPs to support — almost certain given Labour's confidence that they could win next time around.

3. Acropolis-on-Thames

In Brussels, deadlines have a way of slipping. Ask the Greeks.

Under EU law — set out in the treaties — the two-year Article 50 process can be extended with the unanimous agreement of the remaining 27 countries.

Should talks have progressed but failed to reach a deal, pressure will grow for an extension rather than an economically-damaging no deal.

Inside Cabinet, there is a fierce reluctance even to contemplate such a scenario, according to conversations with leading ministers involved in the talks. To go begging for extra time would leave Britain exposed to “blackmail,” said one.

Some MPs believe the real concern among Euroskeptics is that extending talks could create a rolling Greek-style crisis that would grind down the public’s tolerance for Brexit, fueling calls for an in-out referendum on the terms of any deal eventually struck.

4. Tripped up by domestic legislation

Whatever is happening in Brussels, the U.K. government faces a series of legislative hurdles to clear at home.

The EU (Withdrawal) Bill, which seeks to transpose EU regulation into U.K. law, is already stuck in limbo, having cleared second reading in the House of Commons but waiting for the government to allot time for debate at committee stage on the floor of the house after hundreds of amendments were laid by MPs on all sides.

One amendment seeks to give MPs the power to block a no-deal scenario, by forcing the government puts the final agreement with Brussels into a separate piece of legislation, giving MPs the power to debate, amend, or even reject, any final agreement negotiated by Brexit Secretary David Davis.

The provision, which has been laid out by the former Attorney General Dominic Grieve, has the support of around a dozen Conservative MPs — enough, in theory, to defeat the government. Labour's Shadow Brexit Secretary Keir Starmer said Sunday he would throw the party's support behind Grieve's proposal unless the government met a series of tests, including a legally-binding transition, protections for workers and human rights guarantees.

Should Grieve succeed in forcing the government to put the final deal into a new bill, parliament faces a monumental choice in early 2019 — to vote through Britain’s new post-Brexit relationship with the EU (which many MPs will conclude is worse than the status quo) or reject it with unknown consequences.

To reject a painfully-negotiated exit treaty with Brussels — should David Davis succeed in achieving one at all — would almost certainly bring down the government and could, if all else fails, leave Britain sailing off the cliff.

The ensuing general election could be fought on deal or no deal — with no deal meaning remaining in the club.

5. Britain revokes Article 50

A government that really wants to play hardball with Brussels may countenance the unthinkable — withdrawing its intention to leave.

The authors of Article 50 are clear that this is legally possible. It would almost certainly be challenged in the courts nonetheless.

"My clear understanding then and now is that the U.K. or any withdrawing country can reverse Article 50 and withdraw to status quo unilaterally," said one author, the Conservative peer and former MEP Timothy Kirkhope.

By withdrawing the Article 50 notification, Britain buys itself limitless time to pressure Brussels into further concessions before re-triggering the exit clause. Brussels could — as before — refuse to negotiate outside of the Article 50 process, leaving Brexit deadlocked with Britain refusing to leave and Brussels refusing to negotiate.

Such an approach would also spark a political crisis in Westminster amid accusations of betrayal from Euroskeptics. “It is impossible,” said one Cabinet minister. “It just can’t happen. There would be a revolution.”

Another was even more forthright. “People just need to get over it. We’re leaving. Deal with it.”