Theresa May is preparing to put her Brexit deal to a vote for the third time this week.

The House of Commons rejected her deal by 149 votes on Tuesday but Downing Street hopes the new threat of a lengthy Brexit delay will persuade Eurosceptic Conservative MPs to back it.

A lack of support from potential Labour backers, who sense an opportunity to force a softer Brexit if they defeat her deal, means May is probably heading for a third defeat.

Downing Street is already considering a fourth Brexit vote in the coming weeks.

LONDON — Theresa May's Brexit deal was handed a second crushing defeat this week — but it's not dead yet.

The House of Commons voted on Thursday to seek an Article 50 extension which, if approved by European Union member states, will delay the United Kingdom's departure until June 30 if May's deal is approved by Wednesday, and for much longer if it isn't. Brexit was originally scheduled to take place on March 29.

That means a third vote on the prime minister's deal is going to be held next week, probably on Tuesday, handing pro-Leave MPs who have voted against the deal twice one last chance to avoid a lengthy Brexit delay.

There is a growing belief in Westminster that the prime minister could even attempt to hold a fourth meaningful vote in the coming weeks if it is rejected by a slender enough margin next week.

So what is likely to change?

The Democratic Unionist Party which props up May's government, plus most pro-Brexit Conservatives, voted against the deal on the basis of their continued opposition to the Irish backstop. This is a contentious part of the Withdrawal Agreement designed to avoid the emergence of border checks between Northern Ireland and the Republic.

Downing Street had hoped that Attorney General Geoffrey Cox, the government's most senior law officer, would change his legal opinion on the Irish backstop to clarify that it would not lock the UK into an EU customs union indefinitely. But he failed to do so, meaning MPs opposed to the deal had no basis on which to climb down.

Cox is now ready to change his legal advice by suggesting the UK could exit the backstop under the Vienna Convention which governs international treaties, by suggesting a "change of circumstances" would allow the UK to exit the backstop if the EU made it permanent.

There is widespread scepticism in legal circles that the UK would actually be able to use the mechanism — prominent legal commentator David Allen Green described it as the "legal equivalent" of the conspiracy theory that the Earth is actually flat — but it could provide sufficient cover to MPs looking for a reason to back the deal.

Labour senses a chance to force softer Brexit

The UK Brexit secretary Steve Barclay speaks before a vote on the prime minister's proposed Brexit deal, 12 March, 2019. UK Parliament / Mark Duffy

Seventy-five Tory MPs and 10 DUP MPs opposed the prime minister's deal last week, handing her a crushing 149-vote margin of defeat. Downing Street hopes that it can bring the DUP on board with new assurances on the backstop, and that many of those Tory Eurosceptics who opposed the deal can be persuaded to support it faced with threat of Brexit being delayed by much longer than the three month extension currently on the table.

But there are around 20 die-hard pro-Brexit Tories who are likely to oppose the deal in whatever form it exists, as well as eight Remain-supporting MPs who want to stop Brexit altogether and are are unlikely to support it either.

That means May will still need to rely on the votes of dozens of Labour MPs to win a majority. Just three supported her deal this week, despite her government promising to give millions of pounds to their constituencies.

The problem for the prime minister is that many potential Labour backers — there are around 25 — have a number of demands, including a customs union, and now sense an opportunity win them if May's deal goes down.

David Lidington, the de facto deputy prime minister, said this week that MPs would be given a chance to discuss a new way forward at the beginning of next month, should May fail to get her deal through before then. This could include "indicative votes" on alternatives to May's deal, including a softer Brexit which Labour MPs would be likely to back.

That means May is probably heading for a third defeat on her deal next week, albeit by a slender margin.

Don't rule out a fourth.