Theresa May’s plan to implement Brexit without the authorisation of a vote in parliament would be “a contemporary necessity” rather than a misuse of outdated ancient royal powers, the attorney general said at the start of the most keenly awaited constitutional law case in recent memory.

Opening the government’s submission in front of the 11 justices of the supreme court, Jeremy Wright QC and MP told the highest court in the land that royal prerogative powers were an essential part of the UK’s unwritten constitution and could therefore legitimately be used to trigger article 50.

Wright argued that the high court had been wrong to rule last month that article 50 could only be triggered after a vote in parliament, and said that the EU referendum had been conducted “with the universal expectation that the government would implement the result”.

The attorney general was speaking in front of a Westminster courtroom so crowded with lawyers, journalists and members of the public that some of the barristers and solicitors taking part had had to sit in neighbouring courts and watch the hearing by live video link.

Also present was Gina Miller, the fund manager who is the lead claimant in the case against the government. She was cheered loudly as she entered the supreme court and took a seat behind her legal team on the left-hand side of the court.

Protesters from both sides of the argument appeared outside the court; at 9am, the number of Europhile protesters outside the supreme court far outnumbered the Brexiters. An open top, double-decker bus carrying supporters dressed as judges, in wigs, capes and duelling swords parked outside. They chanted, “Nigel, where are you?”

Inside, Wright argued: “The foreign affairs prerogative is not an ancient relic, but a contemporary necessity. Including the powers to make and withdraw from treaties, it is a fundamental pillar of our constitution as a sovereign state and it is essential to the effective conduct of public business.

“The need [for governments] to maintain control of strategy and policy matters is clear and compelling.” Parliament is sovereign, Wright added, but it had chosen to limit the government’s prerogative powers “sparingly, conscious of the need for executive powers”.

The politically sensitive case has provoked fierce attacks on the judiciary and claims from some newspapers that certain justices harbour private Europhile sympathies. In court, the arguments have exposed gaps in the UK’s unwritten constitution, as rival legal teams draw on different precedent cases and wrench an untested consensus apart.

Wright was followed by James Eadie, the seasoned presenter of government cases in the courts. Developing the government’s arguments following Wright’s opening submission, Eadie said that powers to make war and peace have long been part of the government’s executive powers.

But Eadie ran into a barrage of queries from the justices on the court, challenging the source of the royal prerogative and the limits of its use and peppered his presentation with questions, observations and interruptions.

Referring to the way in which the UK entered what was then the EEC, first Lord Wilson and then Lord Neuberger, president of the supreme court, pointed out that parliament and the government had worked together in 1972. “If entry was the result of a joint effort, should our departure [from the EU] also not be so?” Wilson inquired.

Lord Sumption, another of the justices, intervened to point out that where statutory instruments are normally replaced by parliamentary legislation, prerogative powers are removed only by implication.

Lord Reed asked whether what happened in 1972, when the European Communities Act came into force, should not dictate the means of withdrawal from the EU. Eadie disagreed. “What we are in the process of answering is a contemporary legal question,” the first Treasury counsel shot back.

Precedents summoned in support of the government’s prerogative powers included the De Keyser case, which related to compensation claimed for military occupation of a London hotel during the first world war. In that instance, the government relied on prerogative powers to limit compensation. Also invoked were the commercial failure of Freddie Laker’s Skytrain and a case involving the Fire Brigades Union.

Earlier, the attorney general had said that parliament had not voted to scrap the royal prerogative when it could have done so. The 2010 Constitutional and Reform of Government Act introduced a series of controls on ratification of treaties, “but it did not seek to control the prerogative powers [of the government] to make it withdraw from treaties”, Wright told the packed supreme court.

Parliament had been “fully and consciously” involved in drawing up the referendum legislation, the attorney general said. “Parliament expected the government to act on the answer the people gave.”

“If this is all about standing up for parliament,” Wright continued, “I say parliament can stand up for itself. When it comes to leaving the European Union, parliament has had full capacity and multiple opportunities to restrict the executive’s ordinary ability to begin the article 50 process, and it has not chosen to do so.”

As the case began, Theresa May’s official spokeswoman attacked both the Labour party and Liberal Democrats for failing to swing behind “the UK team” over the issue of Brexit. The comments from No 10 came after the spokeswoman was asked about the suggestion by Jeremy Corbyn and others that they would attempt to amend any bill on article 50 that the government introduced in the event of defeat in the supreme court.

“While others are seeming to make clear that they want to frustrate the will of the British people by slowing down the process of leaving and trying to tie the government’s hand in the negotiation, the government is getting on with respecting what the British people decided and making a success of Brexit,” she said. “If you are backing the UK team, you want them to be able to go into the negotiation and get the best deal possible.”

Before the legal arguments began, the president of the supreme court said none of the parties involved in the case had asked any of the 11 justices to stand down, or recuse themselves, from the hearing. Stressing the court’s determination to assert its impartiality in the proceedings, Lord Neuberger said: “At the direction of the court, the registrar has asked all the parties involved in these proceedings whether they wish to ask any of the justices to stand down,” Neuberger explained.

“Without exception, all parties to the appeal have stated that they have no objection to any of us sitting on this appeal.”

(The Guardian)