If nothing changes, Zac Goldsmith will lose the London mayoral election to Sadiq Khan. His decision to back EU withdrawal won’t help his cause. By affirming his lifelong position, the Conservative candidate has taken a principled stance but a politically foolish one. London is the most pro-EU region of the country and Khan, an unambiguous supporter, can now relentlessly harness that vote (the contest takes place on 5 May, shortly before the referendum on 23 June).

Goldsmith’s decision will aid Khan’s effort to position himself as the true pro-business candidate. The City of London is firmly opposed to Brexit and is satisfied with the safeguards David Cameron has achieved for non-eurozone states. Pro-European groups such as the CBI and London First will put Goldsmith's stance under particular scrutiny.

Labour tweeted in response to his announcement: “No serious candidate for Mayor of London would put the prosperity of our city at risk by voting to leave the EU”. Uncomfortably for Goldsmith, it appears that David Cameron agrees. As he sought to persuade Boris Johnson not to support withdrawal (an effort that has likely failed), Cameron told friends: “I can’t understand why Boris, as leader of the great financial capital, won’t support the City”. It will now be far harder for the PM and the pro-EU George Osborne to campaign alongside Goldsmith.

The consolation for the latter is that he will be on the same side as the incumbent mayor (though he risks looking like a weak imitation). But the blanket coverage that the referendum will now attract will make it harder for him to win. The more attention is drawn away from the mayoral contest, the less likely it is that Goldsmith will be able to overcome Khan’s double-digit polling lead. That he finds himself on the opposite side to Londoners on the defining political question of the day will only lengthen the odds on his candidacy.