The government is facing calls to reveal legal advice on how a post-Brexit plan for Northern Irelandâ€™s border could work.
Environment Secretary Michael Gove wants cabinet ministers to see the full details of the so-called “backstop” that would prevent a hard border.
And Northern Irelandâ€™s DUP - which supports the government in key votes - wants it released in full.
The border is proving the main sticking point in reaching a Brexit deal.
Both sides have agreed to put in place a backstop that would only be triggered if a future trade deal is not in place by the end of 2020 - or if the deal did not ensure a frictionless border.
One option is for the whole of the UK to remain temporarily aligned to the EUâ€™s customs union, avoiding the need for customs checks at the border.
BBC political correspondent Iain Watson said Mr Gove was concerned that legally, under this arrangement, Northern Ireland could have to continue to follow EU rules while the rest of the UK would merely mirror them.
He wants to see the full legal advice, fearing cabinet members will only see a summary before having to decide whether to back a deal with Brussels.
The DUP - whose votes Theresa May relies on to win key votes in the Commons - has gone further, saying the legal advice should be released to the public.
DUP chief whip Sir Jeffrey Donaldson told BBC Radio 4â€™s Today programme that it was “in the public interest that we understand fully what is happening”.
“It affects the whole of the United Kingdom and therefore shouldnâ€™t just be the DUP that sees the advice or the government,” he said.
“If the House of Commons are going to have a meaningful vote on a dealâ€¦. upon which this legal advice is very, very important the I think people are entitled to know what that legal advice is.”
Downing Street said it does not comment on legal advice.