London's transportation regulator has refused to renew Uber's license to operate in the U.K. capital, saying the company is not "fit and proper" to run a private ride-hiring service. The city says Uber's systems repeatedly allowed unauthorized drivers to pick up passengers.

"We think this decision is wrong and we will appeal," Uber's U.K. division says, adding that 3.5 million people use Uber's app and services in London.

Transport for London says Uber's computer systems "allowed unauthorized drivers to upload their photos to other Uber driver accounts" and take on customers as if they were the Uber driver, something the regulator says happened on "at least 14,000 trips."

All of those rides, according to Transport for London, were uninsured as a result of the drivers' ability to manipulate Uber's systems. It adds that some of those drivers were unlicensed — and in one case, a driver whose license had been revoked by the agency used the security gap to pick up new passengers.

Identifying another problem, the regulator says former Uber drivers who had been dismissed or suspended were able to elude punishment simply by creating a new Uber account and carrying passengers again.

"Uber has 21 days to lodge an appeal and can continue to operate during any appeals process," NPR's Frank Langfitt reports from London. "Uber remains a popular option in London, particularly as the city's famous black cabs charge considerably more."

London's regulator has previously moved to cancel Uber's license, most notably in 2017. A judge overruled the agency's decision in the summer of 2018 — but then reissued the license for only 15 months, to see if the company made good on its promises to change. At the time, Transport for London said Uber's methods for handling background checks and reporting criminal offenses were flawed. It also accused the company's drivers of using software called "Greyball" to connect with users in cities where the app and ride service are banned.

Uber says it has "fundamentally changed our business over the last 2 years" to adapt to regulators' concerns. But while Transport for London acknowledges some positive changes, it says the company hasn't done enough. And the agency says it's concerned that "Uber's systems seem to have been comparatively easily manipulated."