The outcome of Brexit now depends on Geoffrey Cox, Arlene Foster and Nigel Dodds

The attorney general and the DUP’s leaders will decide our future – because, despite talk of Tory opponents of the prime minister’s deal ‘softening’, its fate is still in the balance

We are now 10 days from the decisive vote on leaving the EU. On Tuesday 12 March the House of Commons will vote again on Theresa May’s Brexit deal – this time with a legally binding explanatory document attached.

If MPs approve the deal, Britain could still leave the EU on 29 March, as planned. Parliament would have to pass a bill to put the terms of the withdrawal agreement into law, but that can be done in double quick time if there is a majority for it.

If they don’t, Brexit is likely to be delayed. This week the prime minister bowed to reality and accepted that the House of Commons would decide what would happen next. First, they would vote on a proposal to leave without agreement. That would be defeated by a large majority. Then they would vote on whether May should ask the EU to postpone Brexit.

It is conceivable that MPs would vote against all three options, in which case the second one, a no-deal Brexit, would miraculously reappear on the table, like a magic trick gone wrong. But as that option has the least support in parliament, I think MPs would vote again on the deal or on delaying Brexit until one of them succeeded.

There is also the possible complication of the rest of the EU refusing to agree to extend the Article 50 deadline. That is a decision that has to be taken unanimously, which means any member state could block it. I think that is unlikely, despite Emmanuel Macron, the French president, saying on Wednesday: “We would in no way accept an extension without a clear objective.”

But if the EU did refuse an extension, parliament would face a starker choice between the prime minister’s deal and cancelling Brexit altogether, which the European Court of Justice has confirmed the UK has a unilateral right to do. Then the deal would be more likely to go through.

So there are still uncertainties about what would happen if the deal is rejected in 10 days’ time, but we now have a clearer idea of what success for Theresa May would look like.

I think we can say with some certainty that the prime minister needs the support of the Democratic Unionist Party to get her deal through.

I have been tracking for a while how MPs might vote if forced to choose between her deal and delaying Brexit. So far, I have counted 35 Labour or ex-Labour MPs who are opposed to a second referendum or who have otherwise said postponing Brexit is unacceptable. Only four of them voted for the prime minister’s deal in January (Ian Austin, Kevin Barron, Frank Field and John Mann). Many of the rest are rude about May’s deal and say they won’t vote for it, but I think most of them will if the alternative is a delay that means Britain might stay in the EU.

What is more, there are other Labour MPs who have not gone public yet who might join them. Only this week, for example, Julie Cooper, MP for Burnley, who hadn’t been on my list, announced: “I have no intention of voting for a second referendum.”

By my calculations, that means May has to reduce the Tory irreconcilables to about 20 if the DUP’s 10 MPs continue to oppose her deal. I’m defining irreconcilables as MPs who want to leave without a deal and who, if they can’t have that, will still vote against her deal, even if it means staying in the EU. They may have failed but at least they will be pure.

The problem for the prime minister is that her advisers think there about 30 of them. That is why I don’t think she can get her deal through without the DUP. If the DUP is satisfied with the assurances given by Geoffrey Cox, the attorney general, most Tory MPs will reluctantly vote for the deal. Which means Arlene Foster, the DUP leader, Nigel Dodds, her leader in Westminster, and Cox, who is negotiating the legally binding addendum to the deal, could decide Brexit.

Cox had a meeting with DUP leaders on Wednesday night, and a source told James Forsyth of The Spectator that the government is “not there yet” on persuading the DUP to come on board. But they would say that, wouldn’t they?

I don’t see how Cox could come up with a form of words on the Irish border that would bridge the gap between the EU27 and the DUP. But, and I have no inside information on this, it would be astonishing if the DUP were not also engaged in negotiations with the prime minister over the next stage of their supply and confidence agreement. The two-year commitment to extra public spending in Northern Ireland, which was signed after the 2017 election, expires in the summer.

There is one part of the country, at least, that can expect a Brexit dividend.