Lady Brenda Hale said a "simple Act of parliament" might not be sufficient to trigger Article 50, which would formally set Britain's divorce proceedings in motion.
Lady Hale has said that the 1972 Act that took Britain into the EU might have to be fully replaced before invoking Article 50.
Supreme Court judge Lady Brenda Hale says “comprehensive” legislation is required to trigger Brexit, potentially delaying Article 50— Arthur O'Connor (@Chantepoule) November 15, 2016
"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 said.
"Another question is whether it would be enough for a simple act of parliament to authorize the government to give notice, or whether it would have to be a comprehensive replacement of the 1972 act," she said during a speech to law students in Kuala Lumpur that were later published online.
Brexiteers Warn of 'Constitutional Crisis'
Lady Hale's comments outlining the potential legal complexities of Brexit comes as the Supreme court prepares to hear a government challenge to a High Court ruling that stipulated MPs must approve the triggering of Article 50.
While arguing that she was merely setting out the potential legal arguments surrounding the UK's exit from the bloc, Lady Hale's comments led to a furious reaction from pro-Brexit MPs, with former cabinet minister Iain Duncan Smith warning such public comments from judges could lead to a "constitutional crisis."
Lady Hale one of the Supreme Court judges hearing the appeal against the legal challenge to #Brexit says vote was 'not legally binding'— Diane James (@DianeJamesMEP) November 15, 2016
"This is a very big step. If they were to do this it's a constitutional crisis. What the judges will decide on at the supreme court is whether or not the government can use its executive powers to trigger article 50," he said.
"It is not their job to tell parliament… how they should go about that business, that's for parliament to decide."
However a supreme court spokesperson dismissed the criticism of the judge, who will one of 11 to hear next month's government appeal.
"Lady Hale was simply presenting the arguments from both sides of the Article 50 appeal in an impartial way for an audience of law students, as part of a wider lecture on constitutional law."
"One of the questions raised in these proceedings is what form of legislation would be necessary for Parliament to be able to lawfully trigger Article 50, if the government loses its appeal.
"A number of politicians have raised the same question. Though it was not dealt with explicitly in the High Court judgment, it is not a new issue. In no way was Lady Hale offering a view on what the likely outcome might be."
Two Year Delay?
The UK's government's defeat in the High Court over MPs' right to approve the triggering of Article 50 has led to increased debate about the sheer complexity of extracting the Britain the EU.
#Brexitplan Article 50 2017, last contribution EU budget 2018, movement controls apply to EU nationals, Reciprocal Tariffs 2019. Simple!— Greg Aitken (@Greg_Aitken) November 15, 2016
If the court rules that all existing EU legislation must be replaced before starting the divorce process, some estimate that this will delay the UK's exit from the EU by up to two years, blowing apart prime minister May's plans to trigger Article 50 by the end of March next year and officially leave the union by the spring of 2019.
While the major UK opposition parties have said they won't bloc the EU leaving process, some have warned they will try to force May to set out her negotiating terms before passing any relevant Brexit legislation.