UK Supreme Court judges were heading in for a third day of a four-day hearing as they considered whether government or Parliament has the power to lead the country out of the European Union.
Prime Minister Theresa May plans to launch EU exit talks by the end of March using ancient powers known as royal prerogative, which enable decisions about joining or leaving international treaties to be made without a parliamentary vote.
Several claimants went to court to argue that leaving the EU would remove some of their rights, including free movement within the bloc, and that such an action could not be taken without legislation in Parliament.
Last month the High Court agreed that the government cannot trigger Article 50 of the EU's key treaty, launching two years of exit negotiations, without lawmakers' support.
The government is challenging that decision at the country's top court - leaving the 11 justices to rule on the balance of power between the legislature and the executive.