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.
Other “serious breaches” also occurred, including several “insurance-related issues”.
TfL pledged to “closely scrutinise” Uber during the appeal process.
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.”
He added: “Over the last two months we have audited every driver in London and further strengthened our processes.
“We have robust systems and checks in place to confirm the identity of drivers and will soon be introducing a new facial matching process, which we believe is a first in London taxi and private hire.”
Mayor of London Sadiq Khan said: “I know this decision may be unpopular with Uber users, but their safety is the paramount concern.
“Regulations are there to keep Londoners safe, and fully complying with TfL’s strict standards is essential if private hire operators want a licence to operate in London.”
Uber was granted a 15-month licence by a judge in June 2018 after it successfully appealed against a TfL decision in September 2017 not to renew its licence amid safety fears.
The firm was handed a two-month extension in September.