Allowing ministers to trigger Brexit was Parliament's "clear expectation" when it agreed to an EU referendum, the Supreme Court has been told.
Attorney General Jeremy Wright said the government would be making "lawful" use of "fundamental" powers by triggering Article 50 of the Lisbon Treaty.
He was speaking at a legal hearing into whether Parliament's consent was needed before Brexit negotiations begin.
The High Court ruled in October that MPs did need to give the go ahead.
The government is appealing against that decision, with the hearing expected to last four days and a verdict due in January. The first day has now concluded, with the case set to resume at 10:15 GMT on Tuesday.
The outcome will have implications for Theresa May's strategy for EU exit, but it is not a court case on whether or not Brexit actually takes place.
In the first session at the Supreme Court:
- Court president Lord Neuberger said the judges would consider issues impartially and decide the case according to the law
- The government set out why it thinks it should be able to use "prerogative powers" to trigger Brexit
- Ministers have the power to make or unmake treaties, it said
- It said the powers were not a "relic" but a key part of the constitution
- Parliament could have chosen to restrict ministers' power to act but had chosen not to, it said
At the start of the hearing, Supreme Court president Lord Neuberger said the justices were aware of the public interest in the case and the "strong feelings associated with the many wider political questions surrounding the United Kingdom's departure from the European Union".
But he added: "This appeal is concerned with legal issues, and, as judges, our duty is to consider those issues impartially, and to decide the case according to the law."
He also said some of the people involved in the case had received "threats of serious violence and unpleasant abuse in emails and other electronic communications", warning anyone that such behaviour "undermines the rule of law".
The UK voted to leave the EU, by a margin of 51.9% to 48.1%, in a referendum in June.
The prime minister has said she intends to officially notify the rest of the EU of the UK's intention to leave - beginning two years of talks over the terms of separation - by the end of March.
But campaigners, led by investment manager Gina Miller and hairdresser Deir Dos Santos, insist that decision can only be taken by Parliament.
The BBC's Clive Coleman said it was an "extraordinary" atmosphere at the Supreme Court in Westminster, with people queuing to get in from early morning, demonstrators dressed as judges and the world's press gathered outside.
The BBC's legal correspondent Clive Coleman summarises the points at stake
- The case is about whether the law means that the government needs the authority of Parliament to trigger the process for the UK to leave the EU
- The government argues it can start the Article 50 process using "prerogative powers", a remnant of the era of all-powerful kings and queens
- Justices will be ruling on who has the legal power to change the rights of British citizens