The government is challenged to receive legal advice on how a post-Brexit plan could work for the Northern Ireland border.
Environment Minister Michael Gove wants Cabinet ministers to see all the details of the so-called "backstop" that would prevent a hard line.
And Northern Ireland's DUP – which supports the government in important votes – wants it to be fully publicized.
The limit is the main sticking point in a Brexit deal.
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Both sides have agreed to set up a backstop, which will only be triggered if a future trade agreement is not concluded by the end of 2020 – or if the agreement does not guarantee a smooth border.
One option is for the entire UK to be temporarily aligned with the EU Customs Union so that customs checks at the border are not required.
BBC political correspondent Iain Watson said Mr Gove was worried that Northern Ireland might still have to follow EU rules under this agreement, while the rest of the UK would just mirror them.
He wants to see the full legal advice, fearing that cabinet members will only see a summary before deciding whether they want to support a deal with Brussels.
The DUP – whose votes Theresa May uses to win key votes in the House of Commons – has gone further and said the legal council should be made public.
DUP chief whip Sir Jeffrey Donaldson told BBC Radio 4's Today that it was "in the public's interest that we fully understand what's happening".
"It affects the entire UK and should not be just the DUP that sees the Council or the government," he said.
"If the House of Commons is going to have a meaningful vote on a deal where this council is very, very important, I think people are entitled to know what that council is."
Downing Street said he does not comment on legal advice.