London’s transit authority refused to grant Uber Technologies Inc. a new licence to operate in the city, endangering the ride-hailing company’s future operations in the city.

While the company has made some progress in addressing Transport for London’s concerns, the regulator is still concerned that unauthorized drivers can manipulate the app to carry passengers, it said in a statement Monday. Uber has 21 days to appeal and can continue to operate while a magistrates’ court considers the decision.

TfL is particularly concerned about a vulnerability in the app that lets unauthorized drivers upload their photos to Uber drivers’ accounts, letting them pick up passengers under the guise of being a licensed driver. This infraction occurred in at least 14,000 trips, including some rides with drivers who had their licenses revoked, according to the TfL. Dismissed and suspended drivers were also able to create an Uber account and carry passengers, it said.

London is one of the most lucrative cities for Uber outside of the U.S. Uber shares dropped about 5.8 per cent in early trading before U.S. markets opened.

Uber had been operating on a two-month license that runs out on Monday, the latest extension while TfL reviewed changes the firm was making to the way it operates. The regulator’s refusal to sign off on Uber’s operations throws doubt over whether the company, which has 45,000 licensed London drivers, has any significant future in Britain.

Many drivers “will now face the distress of facing not only unemployment but also crippling debt as they struggle to meet car lease payments,” “The terrible price of Transport for London’s inability to run a stable regulatory regime and Uber’s refusal to play by the rules will be paid for by the most vulnerable workforce in London.”

TfL concluded more than two years ago that Uber wasn’t “fit and proper” to hold a license, saying the company failed to do adequate background checks on drivers and report serious criminal offenses. It also took issue with Uber software called “Greyball” that blocked government officials attempts to catch lawbreaking drivers. London Mayor Sadiq Khan defended the regulator’s decision at the time.

London’s ride-sharing market has changed significantly since then. Chief Executive Officer Dara Khosrowshahi has made a number changes to win over regulators and the public, which ultimately led to the award of a temporary operating permit. The city has also awarded licenses to similar services, such as Ola, ViaVan and Bolt.

But not everybody is happy. Uber has faced a series of lawsuits, including an employments rights case over how its drivers are treated, which it lost.

Jim Kelly, a taxi driver and chair of the London regional branch of workers’ union Unite, said last week that TfL’s measures for regulating Uber and similar companies don’t go far enough. “It’s in Uber’s DNA to undermine regulation and create a race to the bottom in towns and cities where they operate,” he added.