Uber Technologies Inc’s biggest market in Europe is at risk after regulators revoked the ride-hailing service’s license to operate in London for the second time in less than three years, expressing safety concerns.

The London transportation regulator said Uber failed to adequately verify drivers’ identities and safeguard the service for passengers. At least 14,000 trips involved drivers who weren’t who they said they were. One driver found exploiting Uber’s app had already had a private hire license revoked by the regulator after it discovered the person had received a caution for distributing indecent images of children, said a spokesman for the government agency, Transport for London.