Uber has been stripped of its London operating licence by Transport for London (TfL) this morning.
TfL said it reached its decision after "several breaches that placed passengers and their safety at risk" were identified.
There were concerns over the identification of drivers - with Tfl saying it had found "at least 14,000 trips" made by different drivers to one shown on the app.
The company's current licence runs out at 11.59pm tonight, after TfL extended it for just two months in September.
Transport for London (TfL) said it had identified a "pattern of failures" by Uber, including several breaches that placed passengers and their safety at risk.
A statement reads: "Despite addressing some of these issues, TfL does not have confidence that similar issues will not reoccur in the future, which has led it to conclude that the company is not fit and proper at this time."
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."
There are around 45,000 Uber drivers in London.
Uber said a range of new safety features have been introduced to its app in the past two years.
Earlier this month, it launched a system which automatically checks on the well-being of drivers and passengers when a journey is interrupted by a long stop.
It also unveiled a discrimination reporting button on its app, and collaborated with the AA to produce a safety video to educate drivers on topics such as reading the road, speed, space management and how to drop off and pick up passengers safely.
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But this wasn't enough for TfL.
"While Uber has worked to address these issues, they highlight the potential safety risk to passengers of weak systems and processes," TfL said in a statement.
Uber's regional general manager for northern and eastern Europe Jamie Heywood said: "TfL's decision not to renew Uber's licence in London is extraordinary and wrong, and we will appeal.
"We have fundamentally changed our business over the last two years and are setting the standard on safety. TfL found us to be a fit and proper operator just two months ago, and we continue to go above and beyond.
"On behalf of the 3.5 million riders and 45,000 licensed drivers who depend on Uber in London, we will continue to operate as normal and will do everything we can to work with TfL to resolve this situation."
It's not the first time TFL has stripped Uber of its licence . Back in 2017, TfL concluded Uber was "not fit and proper to hold a private hire operator licence".
The taxi firm appealed, and was eventually granted a 15-month licence by a judge in June 2018 .
It is likely the firm will be allowed to continue operating while any appeals are pending. Uber has 21 days to appeal and then the case goes to magistrate.