Rishi Sunak is once again facing an unhelpful intervention from one of his predecessors. As the Prime Minister attempts to finalise a deal on the Northern Ireland Protocol – spending the past few days meeting both with the DUP and the president of the European Commission – Boris Johnson has issued a warning. A source close to Johnson told the Sunday Telegraph that ditching the Northern Ireland Protocol Bill in favour of a new Brexit deal would be a ‘great mistake’. Speaking this morning on the BBC, government minister Penny Mordaunt sought to downplay the comment – describing it as ‘not an entirely unhelpful intervention’.

Some MPs supportive of Sunak believe he should leave the issue alone, rather than risk a fight with the right of the party

The intervention is focussed on Sunak’s method of negotiating with Brussels. While Johnson is the one who signed up to deal in the first place, insisting that there would be no checks on goods, he later acknowledged that the arrangement did not work for Northern Ireland and the Union. To remedy this, he introduced the Northern Ireland Protocol Bill, which would allow the UK to unilaterally rip up parts of the agreement. The Bill angered Brussels, with EU figures threatening legal action and retaliation if it became law. On the UK side, it was viewed as a way to pile pressure on the EU – even if it was likely to face a backlash in the House of Lords. Sunak, however, has paused the Bill while he tries to agree changes to the Protocol with Brussels. If an agreement is made – which could be announced as early as this coming week – the understanding is that the government would shelve the bill entirely.

With no deal confirmed (Sunak stressed on Saturday that nothing had been finalised), Johnson’s intervention is effectively a warning shot. It suggests that the former prime minister is willing to speak out if he decides the new arrangements do not pass muster. The most significant issue here is the role of the ECJ. While Sunak’s talks with the DUP were amicable, the party has warned that the proposed deal risks not going far enough in resolving its concerns on the ‘democratic deficit’.

As I have reported previously, there has long been concern in No. 10 that Johnson could speak out on the Protocol – and another former prime minister, Liz Truss, could join him in doing so. It’s one of the reasons some MPs supportive of Sunak believe he should leave the issue alone, rather than risk a fight with the right of the party. While a new deal does not require primary legislation – so there is not necessarily a vote – MPs tend to find a way to hold one if the will is there.

The risk for Sunak is that he opens a sore with the right of the party, making governing harder and potentially allowing Johnson to gather supporters should he fancy a comeback. As Johnson loyalist and former Brexit minister James Duddridge puts it: ‘The PM would be unwise to put his own neck on the chopping block. It won’t just be 28 “spartans” voting against this, it will be a large number of Brexiteers, plus a number who don’t see him as the future, plus a number who perhaps were pro-Remain but think Brexit needs to be delivered.’

Sunak ultimately believes it is important to resolve the situation in Northern Ireland. But as Johnson’s comments show, he risks a fight with his party that could grow out of his control.