Theresa May's pledge to trigger Article 50 by the end of March 2017 appears to have suffered another setback after a Supreme Court judge said a "simple Act of Parliament" may not be enough to start Britain's divorce proceedings from the EU.
Lady Brenda Hale said a "comprehensive replacement" of the European Communities Act, which took Britain into the EU, may be required instead. This means the prime minister could be required to repeal and then replace legislation in the 1972 Act, potentially delaying Brexit by two more years, according to the Independent.
Speaking to lawyers in the Malaysian capital, Kuala Lumpur, Hale said: "The argument is that the European Communities Act 1972 grants rights to individuals and others which will automatically be lost if the Treaties cease to apply.
"Such a result, it is said, can only be achieved by an Act of Parliament," she added. "Another question is whether it would be enough for a simple Act of Parliament to authorise the government to give notice, or whether it would have to be a comprehensive replacement for the 1972 Act."
May has promised a "Great Repeal Bill" to end the authority of EU law – something she could be forced to bring forward if she is required to repeal and replace legislation in the 1972 Act before invoking Article 50.
The judge also added that the EU referendum result is advisory rather than legally binding. "As is well known, the referendum on whether the United Kingdom should leave or remain in the European Union (EU) produced a majority of 51.8% in favour of leaving," said Hale. "But that referendum was not legally binding on Parliament."
On 3 November, the High Court ruled that Parliament must vote on Article 50 before it can be triggered. The government is appealing the decision to the Supreme Court, which will settle the most important British constitutional conundrum in years.
Lady Hale will be one of 11 judges to decide the matter in a four-day hearing which is set to begin next month on 5 December.
Her comments have been criticised by pro-Brexit MPs, with Tory MP Jacob Rees-Mogg describing them as "remarkable".
"If their lordships have decided their verdict already they might let us know in the supreme court in Britain, not in Malaysia," said Ukip's Douglas Carswell. "And if their lordships are going to insist that Article 50 can only be triggered by a substantive piece of legislation I think it will undermine what confidence people have in our democracy," he added.