The Northern Ireland Protocol Bill will become law, unless the new prime minister lets it fail

The Northern Ireland Protocol Bill last night moved a step closer to becoming law.

If this crucial legislation becomes law, then Boris Johnson will have moved a considerable distance to improve his legacy as prime minister.

We can go back and forth over why he agreed an Irish Sea border in 2019, but the bill — while far from perfect — will help to mitigate (albeit not remove) that internal UK barrier. It is the minimum that any unionist should accept.

With each stage of the bill’s movement through the House of Commons, it becomes clear that there is not going to be a major Tory rebellion over it.

Thus it becomes harder for any of the Conservative candidates for 10 Downing Street to withdraw their support for the legislation.

It also becomes harder for the House of Lords to thwart it.

The latter obstacle, however, is a greater potential problem if the incoming Tory leader lacks enthusiasm for the bill.

We know that none of the candidates are likely to commit to abandoning the legislation during the remainder of the leadership contest. But what if they go cool on it having won the race, as they turn their attention to mending fences with the European Union?

It is therefore important to flush out now exactly what the Tory hopefuls believe on this crucial matter, so that they cannot easily walk away from vague pledges of support for the overhaul of the protocol.

Worryingly, of the three candidates most likely to prevail in the race — Rishi Sunak, Penny Mordaunt and Liz Truss — only the latter has clear enthusiasm for the bill.

Mr Sunak is reported to have wanted to prioritise good relations with Brussels over unfettered trade within the UK. He is a talented politician but on that policy alone he should be precluded from the top job, unless he starts to speak explicitly about the Irish Sea border and shows an understanding of why it is so unacceptable and how he proposes to remove it.