The fate of Boris Johnson’s premiership will be determined by Nigel Farage and the Brexit party. Even if a Brexit deal can be agreed, another extension to the deadline of 31 October still seems possible. If the can is kicked down the road, the question of how Farage’s voters will react is key.

Without the support of Brexit party voters, Boris Johnson could wake after the next election to find himself and his party still trapped in a hung parliament. But if he wins over half of Farage's supporters, while the Remain camp is divided between Labour and the Liberal Democrats, then he could land nearly 350 seats and a comfortable majority. Win over three-quarters and Johnson enters landslide territory.

Such calculations depend heavily on how Brexit party voters will react to a further Brexit extension – or, indeed, the nature of any deal agreed between the EU and Britain. There are two schools of thought. The most popular contends that when Johnson’s deal fails and he is forced via the Benn Act to agree to an extension then both he and the Conservative party will suffer a mass exodus of support.

Disillusioned by the failure of yet another Conservative leader to deliver Brexit, a large chunk of the six in ten Leavers who remain with Johnson will defect en masse to the Brexit party, joining the one in four Leavers who already sit with the ‘hard’ Brexiteers.

Only last week, Farage sought to woo more of them over by taking out full-page adverts in newspapers, warning Leavers that the Brexit choice is ultimately binary: it’s either a bad deal with Boris and the establishment or a ‘clean break’.

Were such an exodus to happen, then it would not only slash the healthy double-digit leads that the Conservative party has been enjoying in the latest polls but would most likely ruin Johnson’s hopes of a strong majority at the next election.

But there is another school of thought. This points to how Boris Johnson, Dominic Cummings and Number 10 are already busily cultivating a binary choice of their own and one that will have much more power than Farage’s. Does Britain want to be led by Jeremy Corbyn or Boris Johnson?

As in 2015, when Farage’s Ukip voters were relentlessly warned that the choice was between David Cameron or Ed Miliband’s Labour-SNP coalition, Brexit party supporters are about to be bombarded with an updated version. You could vote for Nigel Farage and risk losing Brexit altogether as a Remain alliance, led by a left-wing Marxist, takes control. Or you could vote for Boris Johnson and give him the majority that he needs to ‘Get Brexit Done’.

Look at the polls and it is clear how Brexit party supporters will fold. When they are asked who would make the best Prime Minister, Boris Johnson or Jeremy Corbyn, Brexit party voters break by a margin of 84 per cent to one per cent in favour of Johnson. Similarly, last week Lord Ashcroft asked them whether they would like to see a Conservative government led by Johnson or a Labour government led by Corbyn. A striking 94 per cent opted for a Johnson-led government.

Brexit party voters are, contrary to what is claimed in some newspapers, instinctively on side with Johnson. Look, for example, at the latest Opinium poll. A striking 78 per cent of Brexit party supporters approve of how Boris Johnson is handling Brexit, 74 per cent say they approve of his job as Prime Minister and nearly 60 per cent feel more positive towards him since the Conservative conference.

Such numbers also arrived after an awkward moment for Farage who, after demanding that Johnson resign following the verdict of the Supreme Court, faced criticism on social media from his own Brexit party voters for not standing by a bruised but still pro-Brexit Prime Minister.

Even as Boris Johnson’s premiership came unstuck through the ruling of the Supreme Court and the departure of some of his own MPs, the numbers suggest that he has continued to drill into the Brexit party vote. Back in July, just before Johnson became Prime Minister, 30 per cent of people who had voted Conservative in 2017 had decamped to Farage’s party. By this week, that number has been cut in half. It is now just 16 per cent.

This suggests that Number 10’s strategy of presenting the Prime Minister as a sincere but embattled Brexiteer embroiled in a ‘David versus Goliath’ struggle against a hostile, pro-Remain establishment is cutting through to Brexit party voters who really matter. With this in mind it is worth noting that this week more than 60 per cent of them said that using the word ‘surrender’ was acceptable.

Lots of things could still go wrong for Johnson. Most Brexit party voters are firmly committed ‘No Dealers’ who will spend the next election campaign evaluating Johnson’s sincerity when he talks about putting no deal back on the table.

They are also voters who, as Lord Ashcroft’s polling makes clear, are the most anxious about the economic winds of globalisation.

Farage is basically a Thatcherite but he still talks in a way that addresses their more protectionist and interventionist tendencies. Johnson will need to do the same, so his move on regional inequality and the minimum wage is a good start. He will also need to spend a lot more time on the ground, in Brexit party territory. Brexit is the biggest part of the puzzle but it is not the only part.

Assuming that Johnson can do all of this, then it seems likely that he will not just retain his current support but could also squeeze the Brexit party vote even further.

If he manages to do this then the prize will be what he has always wanted: a big majority, and a long premiership, of his own.