The transport authority said one main issue was a flaw in Uber’s system that let unauthorized drivers sneak onto it. The drivers sidestepped rules by colluding with authorized drivers to pick up riders under their account. At least 14,000 trips were conducted by at least 43 drivers using the workaround.

“This means all the journeys were uninsured and some passenger journeys took place with unlicensed drivers,” Transport for London said.

This practice, known as “account spoofing,” is a challenge for gig-economy platforms to police. Food delivery companies have also seen people working under the accounts of others to sidestep policies.

London officials cited other safety deficiencies at Uber, including instances when dismissed or suspended drivers were able to create another account. Transport for London found several examples in which drivers did not have the correct insurance. The regulators said that because of the volume of problems, they had lost faith in Uber’s ability to improve.

Uber said in a statement that it would operate as normal while it appealed the decision, which it called “extraordinary and wrong.” The company said it first notified regulators about the unauthorized drivers in May and had since closed the loophole.

“We understand we’re held to a high bar, as we should be,” Dara Khosrowshahi, Uber’s chief executive, said on Twitter. “But this TfL decision is just wrong. Over the last 2 years we have fundamentally changed how we operate in London. We have come very far — and we will keep going, for the millions of drivers and riders who rely on us.”

Mr. Khosrowshahi joined Uber in 2017 promising to clean up the company’s culture, which was known for making growth its top priority and flouting government regulation. But Uber has continued to grapple with criticism for safety and not doing enough to protect both riders and drivers.