Uber has lost its license to operate in London, a major blow given that the UK capital with some 3.5 million users is one of the ride-hailing app's biggest markets globally.
The city's regulator, Transport for London, said it had identified a "pattern of failures" by the service that put passengers' safety at risk and was refusing to renew Uber's license, which expires Monday.
TfL said it found unauthorized drivers using the accounts of approved Uber drivers and picking up passengers using vehicles they weren't registered to drive.
The regulator said these fraudulent drivers had conducted 14,000 trips. All the trips were uninsured, and some of the unauthorized drivers had not been licensed at all by the regulator. In one instance, one fraudulent driver had had their license revoked.
TfL said it also found drivers who had been suspended from Uber's platform were still able to create an account and start driving again. It also identified cases in which drivers didn't have the right insurance in place.
The regulator added that Uber had taken steps to fix these issues but said it was "a concern that Uber's systems seem to have been comparatively easily manipulated."
"While we recognize Uber has made improvements, it is unacceptable that Uber has allowed passengers to get into minicabs with drivers who are potentially unlicensed and uninsured," TfL's licensing director, Helen Chapman, said in a statement.
"It is clearly concerning that these issues arose, but it is also concerning that we cannot be confident that similar issues won't happen again in future."
Mayor Sadiq Khan, who oversees London's transport regulator, said he supported the decision to revoke Uber's license.
"I know this decision may be unpopular with Uber users, but their safety is the paramount concern," he said. "Regulations are there to keep Londoners safe, and fully complying with TfL's strict standards is essential if private hire operators want a license to operate in London."