The government had hoped to escape with making public only a summary of the opinion from its lawyers, which is key to claims that the EU will be able to lock the UK into the backstop.
But the middle-way amendment will not be called – prompting Tory Brexiteers to signal they will abstain, making success for Labour’s motion almost inevitable.
There were even suggestions that the government might cave in and allow the motion to pass, to avoid the embarrassment of a defeat in the Commons lobbies.
Unlike normal motions, a “humble address” is binding and the government will be forced to publish – although it could argue the advice does not exist yet, with the negotiations still ongoing.
At stake is the fear of pro-Brexit Tories that the UK risks being shut indefinitely into the EU customs union – unable to sign its own trade deals – unless it can end the backstop unilaterally.
But Ms May has warned her cabinet that the demand must be dropped if the UK is to secure an agreement with the EU within the next few weeks.
Labour then said it would be untenable to keep MPs “in the dark” on the legal advice if they are to have a “meaningful vote” on whether to endorse the prime minister’s departure terms.
Keir Starmer, the party’s Brexit spokesman, said: “At this critical stage, MPs can’t be kept in the dark nor can we risk parliament being bounced into a decision without having all of the facts available.
“Ministers should accept this motion and allow MPs to have an informed debate about the UK’s future relationship with the EU after Brexit.”
Geoffrey Cox, the attorney general, has advised the cabinet on the legal reasoning behind the backstop, but is unlikely to finalise his advice unless and until a deal has been reached in Brussels.
Labour has argued its humble address has been constructed in such a way that the government would have to comply with the demand, even if at a later date.
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