Top lawyer tells the High Court it's 'beyond any doubt' that parliament should vote on Article 50

Theresa May is facing the first serious test of her authority as Prime Minister, with top lawyers on Monday arguing in the High Court that she must defer to parliament on the matter of leaving the European Union.

Three of the United Kingdom's most senior judges on Monday heard arguments about why members of parliament should vote on the triggering of Article 50.

Article 50 begins the official two-year process of Britain leaving the European Union and Theresa May has signalled she will trigger the article in March of next year. However, there is increasing pressure for the triggering to be put to a vote, a process that some commentators say could result in the Brexit process being watered down or even reversed.

Lawyers representing a number of claimants say it would be unlawful for May to initiate Britain's exit from the EU using royal prerogative.

Speaking at the Royal Courts of Justice, Patrick Green QC, representing British expats, said it was "beyond any doubt" that existing law prevents the executive from removing treaties and domestic law without first consulting parliament. Green was referencing the European Communities Act (1972) and European Union Act (2011).

He argued that the former piece of legislation, which saw the UK concede some of its legislative power to the EU, was an example of Parliament conferring powers which "only parliament" could exercise and confer.

This means that triggering Article 50 without first passing an Act of Parliament would be May's government overturning a parliamentary decision without consulting parliament itself, he added.

"There can be no scope for government with a stroke of a pen claiming the power to take away treaties... remove domestic rights and reverse parliament's conferral power without parliamentary approval," he said.

Helen Mountfield QC, representing crowdfunded claim People's Challenge, told the court that triggering Article 50 without parliamentary approval would be contrary to the Scotland Act, which protects Scottish domestic law from alteration without a proper parliamentary process.

"The crown does not have the right to impose or removing rights as a matter of domestic law," she said.

The court room was packed to watch the second day of the historic case unfold. If the judges rule in the claimants' favour, not only would it have a clear impact on how and when Britain leaves the 28-nation bloc, but would totally reshape the country's constitutional landscape.

The government's position was defended by Attorney General Jeremy Wright QC. Wright argued that MPs were told the government intended to use royal prerogative to trigger Article 50 in the result of a Brexit vote and said the referendum legislation did not state that further legislation would be necessary to put the result into effect.

"As a matter of general principle, withdrawal from a treaty is a matter for the Crown... This is totally within the expectation of Parliament... Royal prerogative has not been eroded," he said.

The Attorney General accused the claimants of politicising the legal debate by wanting MPs to be "asked the same question" that was put to the public on June 23.

The case got underway on Thursday and will conclude on Tuesday, with a verdict expected in mid-November. The claimants are expected to launch an appeal to the Supreme Court if the judges rule against them.

The case is being heard against a backdrop of heated debate between Remainers who want Brexit to be subjected to tough parliamentary scrutiny and Brexiteers who accuse Remainers of attempting to subvert the will of the British people.

Some who have attended the case have been verbally abused upon entering the court. Lord Chief Justice opened proceedings by saying this abuse was "wholly wrong" and warned those responsible would receive the "full vigour" of the law.

The case continues.