Uber's licence to operate in London has not been renewed over safety and security concerns, Transport for London (TfL) said.
A spokesman for the transport body said "several breaches that placed passengers and their safety at risk" were identified.
The ride-hailing app firm's existing licence expires at 11.59pm on Monday, but it will be allowed to continue to operate pending an appeal.
TfL's director of licensing, regulation and charging Helen Chapman 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."
TfL found that at least 14,000 trips were made with drivers who were different to the ones shown on the app.
This was due to a system change enabling unauthorised drivers to upload their photographs to legitimate Uber driver accounts, the transport body said.
All of these journeys were uninsured and some took place with unlicensed drivers, including one who had previously had their licence revoked, TfL added.
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."