Theresa May will be forced to publish in full the legal position supporting her Brexit deal following a rebellion by Tory MPs and her DUP allies.
Labour deployed an arcane parliamentary procedure known as a humble address on Tuesday afternoon to obtain the legal advice provided by Attorney General Geoffrey Cox.
Hardline Brexiteer Tory backbenchers and the DUP refused to support the government and vote against Labour’s motion.
Both groups want legal assurances that the proposed Northern Ireland backstop will not keep the UK in a customs union with the EU indefinitely.
In an admission that defeat had become inevitable, the government ordered Tory backbenchers to abstain to avoid an embarrassing losing vote count being recorded.
Pro-EU Tory Anna Soubry accused the prime minister of surrendering to the demands of Brexiteer Tories in the European Research Group (ERG).
“Who is running this country? This government or the ERG?” she said.
The government had earlier attempted to buy off opponents by offering a series of concessions.
David Lidington, the de facto deputy prime minister, agreed to provide MPs with the government’s “legal position” before parliament was asked to vote on any deal.
The Attorney General has also agreed to be questioned in the Commons on his legal opinion once a Withdrawal Agreement has been agreed with the EU.
But Lidington said ministers must be able to receive “full and frank legal advice in confidence”.
He warned if internal government legal advice was routinely made public it would be “less likely to be frank and candid and less likely to be written down”.
Labour’s Shadow Brexit Secretary Keir Starmer said the “unprecedented nature of the Brexit decision” meant it was right the legal advice was published on this occasion.
DUP MP Sammy Wilson tore into the prime minister’s negotiating tactics. “No one has compromised the government more than the government itself in these negotiations. They willing accepted the EU agenda and timetable,” he said.
The defeat bodes ill for May’s ability to win the upcoming parliamentary vote on her Brexit deal as she relies on the DUP for her majority in the Commons.
May said today a “small number” of issues still remain to be resolved as negotiators strive to close the gap in the Brexit talks.
The prime minister briefed the Cabinet on the progress of the discussions as talks continued in Brussels between senior EU and UK officials.