Transport for London said Monday that it will not renew Uber's license, but that won't change anything for riders just yet.

TfL cited a "pattern of failures" that had "placed passenger safety and security at risk."

Uber has already said it will appeal the decision, which will grant the company the opportunity to publicly argue its case in front of a judge. Uber will remain free to operate throughout the appeals process, according to Transport for London. The last time Uber appealed such a move, it took nine months to settle.

"Legislation means that Uber now has 21 days to appeal, during which it can continue to operate pending any appeal and throughout any potential appeals process," TfL wrote in a statement. "Uber may seek to implement changes to demonstrate to a magistrate that it is fit and proper by the time of the appeal."

London is Uber's biggest European market and a key driver of its revenue beyond the U.S. It's faced increased competition in the U.K. capital from the likes of Estonian start-up Bolt and French rival Kapten. Bolt said Monday in a promotional e-mail to users that it has signed up thousands of new customers after Uber's license was revoked.

"TfL's decision not to renew Uber's licence in London is extraordinary and wrong, and we will appeal," Jamie Heywood, Uber's regional general manager for Northern and Eastern Europe, said in a statement. "On behalf of the 3.5 million riders and 45,000 licensed drivers who depend on Uber in London, we will continue to operate as normal and will do everything we can to work with TfL to resolve this situation."

TfL first suspended Uber's license in September 2017, flagging concerns with the company's approach to safety. Uber appealed the decision, and a magistrate overturned TfL's decision in June 2018. In the interim nine-month period, Uber operated as normal.

"It doesn't really change day-to-day operations. Uber will continue to operate. They can continue to bring on new drivers," Meera Joshi, former head of the New York City Taxi and Limousine Commission, said. "There's a magistrate's process, which takes quite a bit of time."

In its announcement, London's transport authority said it had an issue with a change made to Uber's identification systems that allowed unauthorized drivers to upload their photos to other Uber driver accounts.

"This means all the journeys were uninsured and some passenger journeys took place with unlicensed drivers, one of which had previously had their licence revoked by TfL," the regulator said, citing 14,000 such trips.

TfL also claimed that dismissed or suspended Uber drivers were able to create an account and carry passengers. It said "several insurance-related issues" led the regulator to prosecute the company earlier this year.

"We understand we're held to a high bar, as we should be. But this TfL decision is just wrong," wrote Uber CEO Dara Khosrowshahi in a tweet Monday. "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."

Ahead of Monday's announcement, Uber had made a number of safety updates to its app in an apparent attempt to allay regulatory concerns. One tool it added to its platform was a button that users could press to flag discrimination experienced on a trip, while another will send out push notifications in the event that GPS data indicates a car crash may have taken place.

Uber's stock was down less than 1% on Monday morning.

-- CNBC's Ryan Browne and Elizabeth Schulze contributed to this report.