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Uber has lost its license to operate in London, a major blow given the UK capital is one of the ride-hailing app's biggest markets globally with some 3.5 million users.
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 has refused to renew Uber's licence, which expires today.
TfL said it found unauthorized drivers using the accounts of real, 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 where drivers didn't have the right insurance in place.
The regulator added that Uber had taken steps to fix these issues, but said: "It is a concern that Uber's systems seem to have been comparatively easily manipulated."
Licensing director Helen Chapman said in a statement: "While we recognise Uber has made improvements, it is unacceptable that Uber has allowed passengers to get into minicabs with drivers who are potentially unlicensed and uninsured.
"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."
Uber can keep working in London while it appeals
Uber's current license to operate expires today. The company won't halt rides in London immediately, but does risk having to leave the capital if it appeals the regulator's decision and loses. The ride-hailing firm has 21 days to appeal.
Uber said the decision to revoke its license was "extraordinary and wrong" and that it would appeal.
The company said it had put robust checks on driver identification in place, and that it would be introducing a facial matching process.
London mayor Sadiq Khan, who oversees the city'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."
Uber already lost its license once before
This isn't the first time London's regulators have cracked down on Uber, and the company is fighting a number of global regulatory battles.
The ride-hailing app lost its license to operate in London in September 2017, but won back a temporary license after fighting its case in court, changing its UK executives, and demonstrating new safety features in its app.
It appears London's regulator is keen to keep Uber on its toes â even as rival services such as Kapten and Bolt move into the city.
Isobel Asher Hamilton and Adam Bienkov contributed to this report.