Uber has vowed to appeal after Transport for London said it will not be issued with a new licence.
TfL concluded that the minicab app is "not fit and proper" to operate in the capital due to concerns which have "public safety and security implications".
These include its approach to reporting serious criminal offences and how it carries out background checks on its drivers.
But Uber, which is used by 3.5 million people and 40,000 drivers in London, hit back, saying it would appeal and claiming the move "would show the world that, far from being open, London is closed to innovative companies".
Uber will cease operation in the capital on September 30, but the company can continue to operate beyond the expiry date if it decides to appeal the decision.
Mayor of London Sadiq Khan said in a statement: "I want London to be at the forefront of innovation and new technology and to be a natural home for exciting new companies that help Londoners by providing a better and more affordable service.
"However, all companies in London must play by the rules and adhere to the high standards we expect - particularly when it comes to the safety of customers.
"Providing an innovative service must not be at the expense of customer safety and security.
"I fully support TfL's decision - it would be wrong if TfL continued to license Uber if there is any way that this could pose a threat to Londoners' safety and security.
"Any operator of private hire services in London needs to play by the rules."