The Uber ride-hailing service was stripped of its London operating licence on Monday for the second time in just over two years, subject to appeal, after the regulator found a "pattern of failures" on safety and security.
A change to Uber's systems allowed unauthorised drivers to upload their photos to other drivers' accounts, meaning they could pick up passengers as if they were the booked driver, Transport for London (TfL) said. This happened in at least 14,000 trips.
Uber immediately said it would appeal. The process is likely to include court action and could drag on for months, allowing its roughly 45,000 drivers in London to keep operating in the meantime, despite the expiry of Uber's licence on Monday.
Its shares listed in Frankfurt were down 2.3% at 1100 GMT.
TfL said it had "identified a pattern of failures by the company including several breaches that placed passengers and their safety at risk".
"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, whose app-based ordering and demand-sensitive pricing have disrupted business for London's "cabbies" and other established operators in many of the world's cities, said the decision was "extraordinary and wrong".
"Over the last two months we have audited every driver in London and further strengthened our processes," said the firm's boss in northern and eastern Europe, Jamie Heywood.
"We have robust systems and checks in place to confirm the identity of drivers and will soon be introducing a new facial matching process."
Uber and TfL have been at odds since TfL rejected a renewal request in 2017, faulting Uber's approach to reporting serious criminal offences and driver background checks.
The firm, worth over $46 billion since listing its shares in May and says it expects to break even next year, continued to take rides during the last appeals process.
In 2018, after Uber made several changes to its business model, a judge granted it a 15-month probationary licence.
In September, TfL extended that for just two months, far short of the maximum five years, and imposed further conditions covering ride-sharing, insurance and driver document checks.
Ahead of the latest decision, Uber said it would introduce measures such as a "discrimination button" enabling drivers and riders to report abuse, enhanced safety training for drivers, and a direct connection to the emergency services.
The decision comes barely two weeks before a national election and less than six months before Londoners decide whether to re-elect Mayor Sadiq Khan, who is also chairman of TfL.
"I know this decision may be unpopular with Uber users, but their safety is the paramount concern," he said on Monday.
"Fully complying with TfL's strict standards is essential if private hire operators want a licence to operate in London."