Uber has been stripped of its London operating licence by the regulator over a series of breaches that put passenger safety at risk.
Transport for London (TfL) said it has identified "a pattern of failures" by the ride-hailing app firm which led it to conclude that it "is not fit and proper at this time".
Uber said it would the challenge the "extraordinary and wrong" decision.
The company has 21 days to appeal and can continue to operate throughout the process.
A key breach found was a change to Uber's systems that allowed unauthorised drivers to upload their photos to other legitimate driver accounts.
This let them pick up passengers as though they were the booked Uber driver on at least 14,000 trips, which meant all those journeys were uninsured, TfL said.
Some also took place with unlicensed drivers, including one who had previously had their licence revoked, the regulator added.
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Another failing identified allowed dismissed or suspended drivers to create an Uber account and carry passengers.
While TfL recognised Uber had taken steps to try and tackle problems, it was concerned the firm's systems "seem to have been comparatively easily manipulated".
The regulator also cited other "serious breaches" involving unspecified insurance-related issues.
Other "serious breaches" also occurred, including several "insurance-related issues".
TfL pledged to "closely scrutinise" Uber during the appeal process.
Helen Chapman, director of licensing, regulation and charging at TfL, said: "As the regulator of private hire services in London we are required to make a decision today on whether Uber is fit and proper to hold a licence. Safety is our absolute top priority.
"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.
"If they choose to appeal, Uber will have the opportunity to publicly demonstrate to a magistrate whether it has put in place sufficient measures to ensure potential safety risks to passengers are eliminated.
"If they do appeal, Uber can continue to operate and we will closely scrutinise the company to ensure the management has robust controls in place to ensure safety is not compromised during any changes to the app."
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