The convention that legal advice to the cabinet is never published is enforced by the ministerial code. That ban has not halted successive demands for greater political openness.

Theresa May’s decision to prevent cabinet ministers from seeing the full text of the Brexit guidance provided by the attorney general, Geoffrey Cox QC, appears, however, to introduce an additional restriction on the circulation of information at the highest levels of government.

Section 2.12 of the ministerial code explicitly declares that if law officers, such as the attorney general, provide legal advice then cabinet ministers should expect to see the entire text. According to reports the environment secretary, Michael Gove, has asked to see the full advice.

It reads: “When advice from the law officers is included in correspondence between ministers, or in papers for the cabinet or ministerial committees, the conclusions may if necessary be summarised but, if this is done, the complete text of the advice should be attached.”

Downing Street has so far fallen back on the long-standing first line of defence that the government does not discuss legal advice, “or the existence thereof”.

The provision of sensitive legal advice to the cabinet has frequently been controversial. The most notorious dispute focused on guidance given by Lord Goldsmith, attorney general in the run-up to the 2003 Iraq war.

Goldsmith wrote two opinions on the legality of invading Iraq. The full text of the first version written on 7 March 2003 was also not circulated to members of the Labour cabinet. That complete document did not eventually leak out until two years later during the 2005 election.

Its belated publication prompted a similar outcry over breaches of section 2.12 of the ministerial code and allegations that cabinet ministers had been denied vital information before making crucial decisions.

Demands for the attorney general’s decision to be more widely circulated and, if possible published, are growing.

The DUP MP Sir Jeffrey Donaldson told the BBC Today programme: “I think it’s in the public interest we understand fully what’s happening here. It’s because it affects the whole UK, therefore it shouldn’t just be the DUP that sees this advice, or the government.

“If the House of Commons is going to have a meaningful vote on a deal, and upon which this legal advice is very, very important, then I think people are entitled to know what that advice is.”

Labour’s Brexit spokesman, Sir Keir Starmer, said it was “essential MPs are given the opportunity to scrutinise the attorney general’s legal advice before voting on the final deal. The public have the right to know precisely what the cabinet has signed up to and what the implications are for the future.”