British Prime Minister Theresa May must seek parliamentary approval before she begins negotiations on leaving the European Union, the High Court in London ruled.

Lawyers for a group of plaintiffs, led by investment fund manager Gina Miller, successfully argued that May does not have the authority to use an ancient legal privilege to justify her decision to start Brexit negotiations without a parliamentary vote.

"The government does not have power under the crown's prerogative to give notice pursuant to Article 50 for the UK to withdraw from the European Union," the ruling said.

May has said she plans to trigger Article 50 of the Lisbon Treaty - which sets the rules for a two-year negotiating process for a nation leaving the EU - by the end of March.

Speaking outside the High Court, Miller said her case was "about process, not politics."

She urged the government to take the "wise decision of not appealing" the ruling, and instead hold a parliamentary debate on the issue.

Another plaintiff said the ruling was "a victory for parliamentary democracy".

The government, which is expected to appeal to the Supreme Court, had insisted that it does have authority to trigger Article 50 under Britain's prerogative powers, which grant legal privilege to the government on behalf of the sovereign.

Legal experts quoted by British media said clarification was still needed over whether the ruling means that parliament is required to approve new legislation on Brexit.

- dpa