The government faces invitations to reveal legal advice on how a Brexit project could work for the Northern Ireland border.
Environment Minister Michael Gove wants Cabinet Ministers to see the full details of the so-called "backstop" that would hinder a difficult margin.
And the Northern Ireland DUP – which supports the government with major votes – wants to be released in full.
Borders prove to be the main point for a Brexit agreement.
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Both sides agreed to create a backstop that could only be triggered if there is no future trade agreement by the end of 2020 – or if the deal did not provide a frictionless border.
One option is to remain temporarily aligned with the EU customs union in order to avoid the need for customs controls at the border.
BBC political correspondent Iain Watson said Mr Gove is worried that legally, under that regulation, Northern Ireland will have to continue to follow EU rules, while the rest of the United Kingdom will simply reflect them.
He wants to see full legal advice, fearing Cabinet members will only see a summary before deciding whether to support an agreement with Brussels.
The DUP – whose votes Theresa May can rely on to win key votes in Commons – has advanced further, saying that legal advice should be made public.
DUP chief Sir Jeffrey Donaldson said in BBC Radio 4's current program that "it is in the public interest to fully understand what is happening".
"This affects the whole of the UK and therefore it should not just be the DUP that sees the advice or the government," he said.
"If the House of Commons has a substantial vote on an agreement …. on which this legal advice is very, very important, I think that people have the right to know what these legal advice is."
Downing Street said it does not comment on legal advice.