Philip Hammond’s budget misstep on Fixed-Odds Betting Terminals has already lost the government a minister – and now it looks certain to earn it the ignominious accolade of becoming the first administration since James Callaghan’s to lose a vote on its own finance bill.

Some 74 MPs have put their names to an amendment to the budget legislation – which gets its second reading today – demanding that the introduction of a £2 maximum stake for the addictive betting machines is brought forward to next April, rather than October.

That, of course, guarantees defeat for the government. One would expect as much on a cause that unites Tom Watson and Ian Duncan Smith. The likelihood that this would happen was the most bewildering thing about the episode that ended with the resignation of sports minister Tracey Crouch – the government was always going to end up doing what she wanted anyway, but it instead chose the path that invited bad publicity after an otherwise well-received budget and advertised its terminal lack of strategic nous.

Arguably more significant than the defeat itself, however, is the identities of the MPs who will inflict it. 20 Conservative MPs have put their names to the amendment, including nine former ministers. They run the ideological gamut from hard Brexit (Boris Johnson, David Davis, Steve Baker) to continuity Remain (Nicky Morgan, Justine Greening). Just as notable is the presence of Jeffrey Donaldson, the DUP chief whip, and three other unionist MPs.

The FOBT debate predates the Brexit process and it wouldn’t be accurate to describe the coming scrap over the finance bill as a proxy battle (though that is certainly how Johnson, Davis, Baker and Jacob Rees-Mogg will see it). But the alacrity with which MPs from all wings of the Conservative Party have joined the rebellion – and there will be more than those who have put their names to amendments – will remind Downing Street of its most inconvenient parliamentary truth. It does not command a reliable majority in the Commons and nor, increasingly, does it command the loyalty or goodwill of many of its own MPs. Ahead of an impossible vote on her Brexit deal, the message to Theresa May is clear: stick to your current course at your peril.