Early or late? That is the question Labour must now answer. Does it accede to Boris Johnson’s demand for a general election on 15 October? Or does it seek a delay until a much later date in November?

It might seem a small difference – but it could potentially be crucial both to the outcome of the Brexit process and to Labour’s electoral fortunes.

In seeking to hold an election on 15 October, Johnson is hoping that he can do well enough at the ballot box that he will be able to reverse the effect of the anti no-deal legislation due to become law on Monday.

Under that legislation, if Johnson has failed to secure a new agreement with the EU that has the backing of MPs by 19 October – four days later – he will have to secure parliament’s approval if he wishes to leave the EU without an agreement.

If Johnson has won a clear majority in an election on 15 October he will doubtless be able to get MPs to back leaving without a deal. The purpose of the anti no-deal legislation will have been frustrated. By agreeing to the earlier date, Labour will have unwittingly handed Johnson a “get out of jail” card.

The possibility that Johnson might win a majority cannot be ruled out. With an average nine-point lead over Labour in the polls, there is at least a 50% chance that Johnson will do so – and his chances would become even better if he continues to squeeze the Brexit party vote in the way that he has been doing successfully in recent weeks.

In contrast, given how MPs voted this week, it seems highly unlikely that approval for leaving without a deal would be forthcoming from the current House of Commons. And if MPs withhold their approval, the new law says that the prime minister has to seek an extension to the article 50 process.

For a prime minister who has promised to deliver Brexit by 31 October come what may, that would be an utter humiliation. It would likely do severe damage to his party’s prospects in a November election, as the Brexit party would almost certainly hoover up disenchanted Conservative leavers.

Labour might gain seats. It might regain the support of some of the leave voters it has lost in recent months. The party might even have a chance of becoming the largest party in a hung parliament, even if it fails to advance much on the dire 25% of the vote that it currently commands.

Meanwhile, the party’s crucial allies in the coalition of forces that have secured the anti no-deal legislation, the 21 Tories who sacrificed their position as Conservative MPs, would be given a little breathing space to establish how (if they wish) they might rescue their political careers. Keeping them onside will be crucial if they are going to be asked, eventually, to vote against a government proposal to leave without a deal.

That said, there would be a risk in delaying to November. Even though the prime minister’s negotiating position has – in his view at least – been “cut off at the knees”, he might succeed in negotiating a deal with the European Union and go on to secure the support of MPs for that deal.

In that event, Johnson would be able in a November contest to campaign as the prime minister who had (finally) delivered Brexit. Even Nigel Farage might feel his Conservative rival had done a good job and decide not to contest many Conservative seats. By uniting the half of the country that backs leave, the prime minister would be able to dash the hopes of remainers whose votes are divided between Labour and the Liberal Democrats.

In short, in insisting on delaying the election to November, Labour would be placing a bet on Johnson failing to secure a Brexit deal in the next six weeks. Unless Labour MPs think the prime minister is likely to deliver by mid-October – and many commentators would suggest that the talks with the EU have not made much progress so far – then it would seem wiser to wait.

• John Curtice is professor of politics at Strathclyde University and chief commentator at whatukthinks.org/eu