Uber has vowed to appeal after Transport for London said it will not be issued with a new licence and was "not fit and proper" to operate in the capital.
TfL said it took the decision on the grounds of "public safety and security implications".
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".
Tom Elvidge, general manager of Uber in London, said that drivers and clients would be “astounded”.
“By wanting to ban our app from the capital Transport for London and the Mayor have caved in to a small number of people who want to restrict consumer choice. If this decision stands, it will put more than 40,000 licensed drivers out of work and deprive Londoners of a convenient and affordable form of transport,” he said.
“To defend the livelihoods of all those drivers, and the consumer choice of millions of Londoners who use our app, we intend to immediately challenge this in the courts.”
He said that drivers who use Uber are licensed by TfL and have been through the same background checks as black cab drivers.
“We have always followed TfL rules on reporting serious incidents and have a dedicated team who work closely with the Metropolitan Police.”
However, 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."