U.K. Supreme Court judges were presented with starkly contrasting views of Britain's largely unwritten constitution Tuesday, as they considered whether government or Parliament has the power to lead the country out of the European Union.
A top constitutional lawyer opposing the government said the power lay only with the U.K.'s sovereign Parliament, and not the Conservative government.
David Pannick said it would be "quite extraordinary" if rights and duties created by Parliament when it took Britain into the European bloc "could be nullified by ministers."
Government lawyer James Eadie disagreed, warning judges they would fall into a "serious constitutional trap" if they ruled that lawmakers must have a vote before government ministers start EU divorce talks.
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 shouldn't be done 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.
On the second day of a four-day hearing, Pannick, representing financial entrepreneur Gina Miller, said an act of Parliament — the European Communities Act of 1972 — took Britain into the EU's predecessor, the European Economic Community. And so, he said, an act of Parliament was needed to take Britain out of the bloc.
Pannick said the 1972 act created "a new source of rights and duties" — the European bloc.
"The idea that ministers could revoke this fundamental change to our constitutional order, in my submission, is inherently unlikely," he said.
But Eadie said the government has constitutional authority to use the royal prerogative to enact voters' June 23 decision to leave the EU. He argued that Parliament has already had its say, by passing the EU Referendum Act of 2015, which made June's referendum on EU membership possible.
He said if judges ruled against the government, "the courts would be imposing in effect a new control of a most serious kind" on the government's ability to make international treaties.
Eadie said the case against the government was "a serious constitutional trap."
May's plan to trigger Article 50 could be delayed if the Supreme Court rules against the government. The justices are expected to rule in January.
The British leader is already under pressure at home and abroad to reveal — and speed up — her Brexit plans. May has refused to disclose what relationship Britain will seek with the EU once it leaves, saying that would weaken the government's negotiating hand.
In Brussels on Tuesday, the EU's chief Brexit negotiator, Michel Barnier, said parliamentary approvals across Europe and other formal procedures meant that Britain and the bloc might have only 18 months, rather than two years, to strike a deal.
Barnier said Britain would not be allowed to "cherry-pick" what it likes about the EU, noting that the bloc's "four freedoms" — the free movement of goods, services, capital and labor — are "indivisible."
Meanwhile, opposition Labour Party lawmakers forced a Wednesday vote in Britain's House of Commons, calling for the government to offer some clarity by publishing its plan for leaving the EU before it triggers Article 50.
The Commons will also vote on a government amendment saying Parliament "calls on the government to invoke Article 50" by March 31. May's office said the vote does not affect the Supreme Court case.