Transport for London has announced that it will not grant Uber a new licence to operate.
As a regulator for taxi and private hire services in London, TfL is required to make a decision on Uber's "fitness and propriety before its current licence expires".
While it admits that Uber made a number of positive changes and "improvements to its culture, leadership and systems in the period since the Chief Magistrate granted it a licence in June 2018", TfL says safety is its main priority, and that it has identified a "pattern of failures" by the company, including several breaches that put passengers at risk.
In a statement, TfL said: "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."
Uber was granted a two-month licence in September, but a key issue identified was a change to Uber's systems that allowed unauthorised drivers to upload their photos to other Uber driver accounts. This allowed drivers to pick up passengers as though they were the booked driver, which happened at least 14,000 times.
This meant the journeys were uninsured and some journeys took place with unlicensed drivers, one of which had had their licence revoked by Transport for London.
Another issue was the ability for dismissed or suspended drivers to create an Uber account and carry passengers.
TfL said insurance related issues were also a reason to refuse Uber to operate, with TfL prosecuting Uber earlier in the year for permitting the use of vehicles without the correct hire or reward insurance in place.
"This pattern of regulatory breaches led TfL to commission an independent assessment of Uber's ability to prevent incidents of this nature happening again."
Uber has 21 days to appeal, during which time it can continue to operate.
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."
Speaking to The Telegraph, Uber's Regional Manager for Northern and Eastern Europe 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."
The Mayor of London, Sadiq Khan, has expressed support for TfL's decision: "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 filly complying with TfL's strict standards is essential if private hire operators want a licence to operate in London."