Theresa May says that the advice is confidential, though some MPs believe that ministers simply want to avoid admitting the deal binds the UK to EU customs rules.

Attorney General Geoffrey Cox is due to publish a reduced version of the advice at 2pm, in spite of calls from MPs for a full one.

Former Lord Chancellor Lord Falconer, a Labour peer, told BBC Radio 4’s Today programme that the public “have the full right to know” about the legal advice and added that the debate in the House of Commons should “properly be informed” by it.

However, former Solicitor General and Tory peer Lord Garnier told the programme it was a “matter of convention” that the advice was withheld and that government policy was under attack and not legal advice.

Labour’s shadow Brexit Secretary, Sir Keir Starmer, said yesterday that if ministers refused to publish the advice they risked causing a “historic constitutional row”.

He commented: “If they don’t produce [the advice] tomorrow then we will start contempt proceedings, this will be a collision course between the government and Parliament.”