British Prime Minister Theresa May was accused yesterday of misleading MPs over her Brexit deal as her government published legal advice likely to increase opposition to the agreement ahead of a crucial vote next week.

Scottish National Party (SNP) lawmaker Ian Blackford was twice reprimanded by the House of Commons speaker for suggesting May had misled MPs "inadvertently or otherwise", before withdrawing the claim.

May denied the claim and said she had been always clear about her government's dealings about Brexit. But she emphasised neither side wanted this to happen, and repeated that the withdrawal agreement struck with Brussels last month was the only viable option.

May on Tuesday suffered a series of stunning defeats in parliament which threaten her government and ultimately could change the course of Brexit.

She effectively lost her majority in the Commons after the Northern Irish party on which she relies sided with the Labour party to find her ministers in contempt of parliament for failing to publish in full the legal advice on the Brexit deal.

Meanwhile 25 her own Conservative MPs voted with Labour to give the Commons a bigger say in what happens if, as expected, the Brexit deal is voted down on December 11.

The government yesterday finally published the six-page advice from the attorney general to cabinet. It confirms Britain risks remaining "indefinitely" in the so-called backstop, which could keep the whole country in an EU customs union for years after Brexit, while also keeping the province of Northern Ireland in the bloc's single market.

MPs on Tuesday also voted to approve an amendment tabled by Conservative former attorney general Dominic Grieve, which allows parliament to determine what happens if the deal falls.

If May loses the vote next week, the government has 21 days to return to MPs to propose what happens next. Grieve's amendment could allow MPs to amend that statement, raising the possibility they could demand a re-negotiation, a second referendum or even staying in the EU.

There are few people who believe May's deal will survive the vote next Tuesday but the question of what happens next remains wide open.