Attorney General Jeremy Wright has defended Theresa May's decision to use the royal prerogative in her strategy for Brexit.
The Government's top legal officer told the High Court in London the prime minister was legitimately giving effect "to the will of the people".
Campaigners from many walks of life have mounted a historic challenge to argue that Mrs May lacks power to trigger Article 50 of the Lisbon Treaty to leave the European Union without the prior authorisation of Parliament.
Disagreeing, the attorney general told three senior judges resort to Article 50 involved the "proper and well established use of the royal prerogative by the executive".
It had been made available, following the vote in favour of Brexit, "to give effect to the will of the people - wholly within the expectation of Parliament".
The attorney general added that "notification - once given - will not be withdrawn.
"It is our case that Parliament's consent is not required."
The judicial review application, which is being heard by the Lord Chief Justice, Lord Thomas, and two other judges, has been described as one of the most important constitutional cases in generations.
Mrs May announced at the Conservative Party conference that she intends giving an Article 50 notification by the end of March 2017 using the royal prerogative.
Her opponents are arguing she would be pre-empting Parliament and unlawfully removing statutory rights granted to UK citizens under the European Communities Act 1972, which made EU law part of UK law.
Because of the urgency and constitutional importance of the case, any appeal is expected to be heard by the Supreme Court, the highest court in the land, before the end of the year.
Lord Thomas has hit out at people who have threatened businesswoman Gina Miller, an investment fund manager and philanthropist who is at the forefront of the Brexit challenge.
On the second day of the hearing, Lord Thomas said the court had been informed that Ms Miller had received abusive emails and "other communications".
He declared to the packed courtroom: "It is simply wholly wrong for people to be abusive of those who seek to come to the Queen's courts."
And he warned those responsible that they had to appreciate "the full vigour of the law" would be used against them if the conduct continued.
James Eadie QC, also representing the Government, said: "There is a reason there are so many people in court.
"It is because this case has profound constitutional implications and profound political implications.
"It also raises a series of questions about how the British constitution should react in unique circumstances that concern the court and confront the country at this time."
Mr Eadie said there was no written, constitutional formula. He said the claimants in court wanted to use legal principles developed in very different constitutional and legal circumstances to make them fit their argument - "precisely denying the constitutional flexibility that lies at the heart of our constitution".
Denying accusations that Parliamentary sovereignty was being pre-empted, Mr Eadie said it remained supreme and was unaffected by the use of the royal prerogative for Article 50.
This was because Parliament itself had left powers to withdraw from international treaties in the hands of the Crown, to be exercised by ministers through the royal prerogative, even if that affected the rights gained by British citizens through EU membership.
Starting the process of withdrawal from the EU by triggering Article 50 would not of itself change any common law or statutory right enjoyed by citizens.
"Any such changes are a matter for future negotiations, Parliamentary scrutiny, and implementation by legislation," he said.
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