The government has published its Brexit legal advice in full, a day after MPs found it in contempt of UK parliament.
In the six-page letter, Geoffrey Cox said the proposed backstop arrangement with the EU to prevent a hard Irish border could “endure indefinitely”.
The UK, he said, would not be able to “lawfully exit” the arrangement without a subsequent political agreement and this could lead to “stalemate”.
Labour said it showed the “central weakness” of the PM’s Brexit deal.
Speaking at Prime Minister’s Questions, the SNP’s Westminster leader Ian Blackford said the legal advice had been “dragged out” of ministers and accused the PM of “misleading Parliament, inadvertently or otherwise”.
Mrs May rejected this, saying the legal position had been set out clearly.
Ministers agreed to publish the advice after being found in contempt of Parliament by MPs on Tuesday, in one of a series of Brexit defeats for Theresa May.
Mr Cox published a legal overview of the repercussions of Mrs May’s Brexit deal earlier this week but argued that disclosing the full advice would compromise client confidentiality and be against the national interest.
Labour and other opposition parties said ministers had “wilfully” refused to comply with a binding vote in the Commons last month which demanded full disclosure.
The Irish backstop, a key feature of the EU withdrawal agreement, would see the whole of the UK remain in a single customs territory with the EU until their future relationship was sorted out.
The PM has insisted it is a temporary “insurance policy” and neither side want it to come into force.
Its legal status has become a defining issue for many Tory critics of the PM’s deal – who say the UK could be trapped indefinitely because there is no unilateral right to withdraw.
In his letter to the PM, dated 13 November, Mr Cox said signing up to the backstop was a “political decision”.
“Despite statements that it is not intended to be permanent…in international law the protocol would endure indefinitely until a superseding agreement took its place,” he wrote.
“In the absence of a right of termination, there is a legal risk that the UK might become subject to protracted and repeated rounds of negotiations.”
“This risk must be weighed against the political and economic imperative of both sides to reach an agreement that constitutes a politically stable and permanent basis for their future relationship.”