London stripped Uber on Friday of its licence to operate from the end of September in a huge blow to the taxi app that will affect more than 40,000 drivers in one of the world's biggest cities.

Regulator Transport for London (TfL) said Uber's conduct posed risks to public safety and it would not renew its licence when it expires on Sept. 30. Uber has 21 days to appeal and can continue to operate until the appeal process has finished.

"Uber's approach and conduct demonstrate a lack of corporate responsibility in relation to a number of issues which have potential public safety and security implications," TfL said.

Uber, which accounts for a third of private hire vehicles on London's streets, said it would contest the decision.

"Transport for London and the Mayor have caved in to a small number of people who want to restrict consumer choice," said Tom Elvidge, Uber's general manager in London. "We intend to immediately challenge this in the courts."

Uber has been attacked by London's black cab drivers who say it has undercut safety rules and threatened their livelihoods. The U.S. firm has also faced criticism from unions and lawmakers and been embroiled in legal battles over workers' rights.

Uber has endured a tumultuous few months after a string of scandals involving allegations of sexism and bullying at the Silicon Valley start-up that forced out former CEO and co-founder Travis Kalanick.

The app has been forced to quit several countries including Denmark and Hungary and faced regulatory battles in multiple U.S. states and countries around the world.

One of Uber's British competitors in London, Addison Lee, is also awaiting a decision from TfL about a longer-term licence. The company declined to comment on Friday.

London Mayor Sadiq Khan said he backed the decision to reject Uber's application for a new licence.

"All companies in London must play by the rules and adhere to the high standards we expect - particularly when it comes to the safety of customers," he said.

"It would be wrong if TfL continued to license Uber if there is any way that this could pose a threat to Londoners' safety and security."