The success of Brexit is now in the hands of a small group of MPs – and not the ones you might think

What happened this week was that our departure from the EU has almost certainly been delayed. There is still a small chance we could leave in disarray in two weeks’ time, but that requires bad miscalculations by both sides.

So the choice now facing parliament is whether to leave on Theresa May’s terms after a delay of a few weeks, or to ask the EU for a longer delay. The House of Commons is expected to vote on this question on Tuesday, when the prime minister will ask MPs, for the third time, to approve her deal.

The numbers are not in her favour. The first rule of writing a political column that is easy to read is to avoid numbers, but that is not possible this week. So here goes.

First May needs the support of the Democratic Unionist Party, with its 10 MPs. James Forsyth of The Spectator quotes a cabinet minister as saying there is a “60:40” chance that they will back the deal this time. In other words, the deal is basically done but they are still holding out for more spending on schools and hospitals in Northern Ireland.

If the DUP agrees, it will topple the next domino, the pragmatic Conservative rebels who are looking for reasons to change their vote. For example, Esther McVey, the former cabinet minister who voted against the deal this week. She said MPs such as she would have to hold their noses and vote for the deal “if they want Brexit”.

But Theresa May needs 65 MPs to change sides, in addition to the DUP, and there are only 75 Conservative votes available. Of the 75 Conservatives who voted against her deal this week, six want to stop Brexit altogether: Guto Bebb, Justine Greening, Dominic Grieve, Sam Gyimah, Jo Johnson and Phillip Lee. They will vote against the deal come what may.

At the other end of the Tory spectrum, there is a hard core of MPs who sincerely believe that the prime minister’s deal is not just “Remain by another name” but actually worse than remaining in the EU. Their argument is that Britain cannot get out of the backstop – the agreement to keep an open border in Ireland – without the EU’s permission, whereas it can leave the EU just by giving two years’ notice.

It is an argument that overlooks the two years’ notice we have just given the EU before they decided it was better to stay, but I think their logic is that next time Brexit would be done properly because Boris Johnson, a reincarnated cross between Churchill and Thatcher, would be prime minister. On a Chieftain tank. Or something.

Anyway, the point is that there are 20 to 30 of these MPs. Whether or not they include Johnson himself will be one to watch on Tuesday. Some time ago he declared that the prime minister’s deal was worse than staying in the EU, but I don’t know if he still thinks that claim is the best route to No 10. The latest ConHome survey showed rising support for the prime minister’s deal among party members, although they still opposed it by 56 per cent to 40 per cent.

That means that what has always been true is still true. We will not leave the EU unless a significant number of Labour MPs vote for it. So far, only five have done so: Kevin Barron, Caroline Flint and John Mann, plus Ian Austin and Frank Field, who have resigned the Labour whip.

The arithmetic of May’s deal is that, if she gets the DUP, she needs a Labour MP for every irreconcilably Eurosceptic Tory MP who refuses to vote for it. Plus one.

So if there are 20 Tory irreconcilables, she needs 21 Labour MPs, including the five who voted for the deal this week.

I know of a handful of Labour MPs who intend to vote for the deal on Tuesday, and another handful who say they will if it looks as if the prime minister will win – one said: “Why should I get grief from my local party if it doesn’t even go through?” And it was significant that five Labour frontbenchers resigned this week to vote against a referendum. The easy thing would have been to abstain, which was the party line, but they felt strongly enough to give up their posts. That suggests they would be willing to vote for the deal if they thought the alternative was that Brexit would never happen – and if there is a long extension that becomes ever more likely.

Altogether, there are 36 Labour and ex-Labour MPs who either oppose a referendum or say Brexit must be delivered. These are the people who will decide whether or not we leave the EU.

I don’t know if that will be decided on Tuesday night. It is quite possible that, if the prime minister comes close to getting her deal through, she will try again on Wednesday before flying to Brussels for the EU summit on Thursday.

But if she wins, it will be on Labour votes.