If there is to be an election before we leave the European Union, some kind of non-aggression pact between the Tories and the Brexit party is essential. Without it, the risk is all too obvious: that pro-Brexit voters will be divided, allowing pro-Remain candidates to win, even in some constituencies where a clear majority are in favour of leaving.

A case in point is Boris Johnson’s constituency. Uxbridge and South Ruislip is in the London borough of Hillingdon, where 56.37 per cent of votes cast in the 2016 referendum were for Leave. But his majority in 2017 was only 5,034, and if the Brexit party fields a candidate against him — particularly if some of the pro-Remain parties decide to stand down in favour of Labour — there’s a chance he’ll lose.

The ideal pact would be a formal one, endorsed by both party leaders. Nigel Farage would agree not to put up candidates in the vast majority of constituencies, including those with a sitting Conservative MP, and in return Boris would give the Brexit party a clear run in, say, two dozen Leave-voting Labour constituencies where the Tories don’t stand a chance. Such an arrangement would be eminently sensible and practically guarantee we’d exit the EU on 31 October. But I’m not optimistic. Boris will be concerned that an alliance with Farage will drive some Conservative voters into the arms of the Lib Dems, particularly in the 25 per cent of Tory seats where a majority voted Remain, while Farage will be wary of alienating potential Labour defectors. There’s also the fact that Boris would prefer to leave with a deal, whereas Farage is a committed no-dealer. Farage has said he won’t countenance a pact unless Boris unequivocally endorses no deal, something he’s unlikely to do.

But that doesn’t mean the situation is hopeless — not quite. The alternative is what I proposed back in 2013 when I was touting a Conservative--Ukip pact, which is a grass-roots, bottom-up alliance between the supporters of both parties. You don’t need the blessing of the party leaders. You just need to persuade Brexit supporters to vote tactically. That means casting their ballots for the pro--Brexit candidate who has the best chance of winning, regardless of party affiliation. You don’t need it to work in every constituency, either. Rather, you would focus on those seats where the Tory incumbent is in danger of losing to a Remain rival, those where a pro-Brexit Conservative is within spitting distance of taking the seat, and those where the Brexit party candidate has a realistic chance of beating the Remainer incumbent. We’re probably talking about no more than 100 constituencies.

To make this work, supporters of both parties would have to trust the organisation advising them on which way to vote. Ideally, it would be led by a political figure known to be pro-Brexit, but not tainted by association with either the Tories or the Brexit party — perhaps a Labour rebel such as Kate Hoey. Indeed, there’s no reason the alliance shouldn’t support those few Labour MPs who are both pro-Brexit and could be relied upon to vote against any further efforts by the Remainers to obstruct our departure. There would be some wrangling about what data set to use when determining which pro-Brexit candidate is most likely to win in Labour marginals — the 2019 European election results would favour the Brexit party candidate, while the 2017 general election results would favour the Conservative — but I’m sure that could be resolved with the help of a sympathetic psephologist.

Time is of the essence. The other side is quite far along in putting together an almost identical arrangement — the Remain Alliance — and has identified 100 target seats, in some of which the Lib Dems, Greens and Plaid Cymru have agreed a non-aggression pact. It also has proof of concept: the Brecon and Radnorshire by-election, which the Lib Dems won after the Greens and Plaid stood down. No sign of Labour participating, but John McDonnell has already floated the idea of an alliance with the SNP. My side needs to get its skates on.

I hope my pessimism about a formal pact is misplaced. Perhaps Boris and Farage might be able to overcome their mutual mistrust. But in the meantime, we need to start mobilising. If there is an official pact, this grass-roots alliance can help implement it; if there isn’t, it would be the next best thing.

To the barricades, comrades. The great victory we won in 2016 is in danger of being snatched away.