The government is facing calls to reveal legal advice on how a post-Brexit plan for Northern Ireland'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 Ireland'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. EU chief negotiator Michel Barnier said it was "clear that more work is needed".
Both sides have agreed to put in place a backstop, also referred to as an insurance policy, that would only be triggered if a future trade deal is not in place by the end of 2020 - or if this final deal does not ensure a frictionless border. This is because there have been warnings that a return of visible border checks could undermine the peace process in Northern Ireland as well as damaging businesses operating on both sides.
One option for a backstop is for the whole of the UK to remain temporarily aligned to the EU's customs union, avoiding the need for customs checks at the border. 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.