Uber has said it plans to launch an appeal after London's transport regulator stripped the ridesharing company of its operating licence for the second time in just over two years.
Transport for London (TfL) revoked the licence on Monday after finding the app was not "fit and proper" and that a change in the system had endangered users' safety.
According to TfL, the Uber app allows unauthorised drivers to use the accounts of other drivers to carry out trips by letting them upload their pictures to authorised driver accounts.
This happened on at least 14,000 trips across the UK's capital, TfL said.
"It is unacceptable that Uber has allowed passengers to get into minicabs with drivers who are potentially unlicensed and uninsured," TfL's director of licensing, regulation and charging, Helen Chapman, said.
London mayor Sadiq Khan also welcomed the decision on Monday, stressing the safety of Londoners was his "paramount concern".
He said: "I support the decision by TfL's licensing officials today in relation to Uber and completely understand why the decision was taken.
"Keeping Londoners safe is my absolute number one priority, and TfL have identified a pattern of failure by Uber that has directly put passengers' safety at risk."
In response, Uber said it planned to appeal the decision and insisted it had taken steps to change its system and increase safety standards.
"Over the last two months we have audited every driver in London," Uber said in a statement.
"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."