The UK Government's legal argument over who has the right to begin Brexit has come under scrutiny at the first day of the Supreme Court hearing.

Downing Street's appeal is largely based on arguing the Prime Minister has "prerogative powers" to trigger Article 50 and therefore does not need to consult MPs on the matter.

Theresa May has said she will use the legal mechanism before the end of March.

The UK Government is appealing to the Supreme Court after the High Court ruled against it last month.

Government lawyer James Eadie told the court prerogative powers were used to enter the precursor to the European Union in the 1970s.

Then-Prime Minister Ted Heath signed the Treaty of Ascension in January, 1972, ten months later parliament passed the European Communities Act 1972 after 300 hours of debate.

The court asked Mr Eadie to explain how both parliamentary and prerogative powers were required to enter the organisation but not now needed to leave.

Judge Lord Wilson said: "Mr Eadie, you have shown very convincingly that our entry into the EU was a joint effort, the exercise of prerogative power by the executive and the exercise of legislative power by parliament.

"And so, to put very simply, one of the arguments you will have to deal with is if our accession was the result of joint effort should our departure not equally be so?"

Mr Eadie defended the government's argument by saying the prerogative powers were not subsumed by the legislation.

He told the court: "The joint effort as it were in the 1972 Act is a joint effort in the sense that it assumes all the prerogative powers continue to exist and be operated.

"So all this is doing, this act, is not to... put it in the positive, this act is designed, and that's all part one (of the act) does, to deal with transposition (implementing European directives in domestic law).

"It does not authorise, it does not purport to be a joint effort in relation to the going in.

"It simply assumes and is premised on the continued existence of that power and withdrawal therefore is entirely consistent, to put it in my lord's helpful way, is entirely consistent with that framework. When you withdraw you withdraw."

Lord Neuberger, the president of the Supreme Court, said he "feels the force" of Mr Eadie's argument but it was "not quite an answer to Lord Wilson's question".

Lord Carnwath also asked the government lawyer why he was spending so much time addressing the court on the 1972 Act when it had been fundamentally changed by the Treaty of Lisbon in 2008, which created the Article 50 legal mechanism to leave.

Earlier in the day, the attorney general Jeremy Wright, acting on behalf of the government, told the 11 Supreme Court justices that triggering Article 50 "will not be an exercise of the prerogative right on a whim or out of the blue" but was instead part of a process which "parliament has been fully and consciously involved".

Mr Wright said the use of the prerogative powers in these circumstances would be lawful. The prerogative was not "an ancient relic" but is a contemporary "constitutional necessity".

The court will resume on Tuesday for the second day of the four-day hearing.

A verdict is expected to be reached by the court in January.