If attorney general Geoffrey Cox does not publish the advice, Labour will use a humble address motion in the House of Commons to force its release. That's the same procedural device Labour used to force Theresa May's government to publish legal advice on its Brexit deal in November last year.

Speaker John Bercow signalled on Tuesday that he would allow parliamentary time when the Commons sits tomorrow for emergency debates.

It is likely the government would lose any vote on the matter.

The publication of the advice Johnson received on prorogation could include details of whether he was told that suspending Parliament was unlawful. It could also place significant political pressure on the position of Cox.

A source close to Amber Rudd told BuzzFeed News she would vote for Labour's humble address motion so long as it was narrow and focussed on securing the legal advice.