The transportation authorities in London announced on Monday that they had decided not to extend Uber’s taxi operating license, throwing into question whether the company will be able to continue to operate in its most lucrative European market.

The decision will not immediately affect Uber’s presence on London streets, as the company has 21 days to appeal the decision and can continue to operate throughout that time. Uber quickly announced that it would file an appeal.

Transport for London, which regulates taxi and private hire services in the city, announced the decision in a statement, saying that while Uber “has made a number of positive changes and improvement to its culture, leadership and systems,” it had not gone far enough. The license expires at 11:59 p.m. on Monday.

“TfL has identified a pattern of failures by the company including several breaches that placed passengers and their safety at risk,” the statement read.

Uber has been in a battle to retain its license in London for years. The company’s operating style and business model have long been a point of contention for city officials and have put Uber at odds with regulators and drivers of the city’s traditional cabs.

The transport authority said that one of the main issues was a change to Uber’s systems that allowed unauthorized drivers to upload photos to other Uber driver accounts and allowed them to pick up passengers on at least 14,000 trips, which the agency said put passenger safety and security at risk.

Jamie Heywood, Uber’s regional general manager for Northern and Eastern Europe, said in a statement that the company would appeal the decision, which he called “extraordinary and wrong,” and that it would continue to operate as normal.

“We have fundamentally changed our business over the last two years and are setting the standard on safety,” he said. “TfL found us to be a fit and proper operator just two months ago, and we continue to go above and beyond.”

Transport for London first revoked Uber’s London license in 2017, though the company was able to continue operating during the appeals process. In June 2018, Uber won an appeal to regain its taxi license in London after agreeing to stricter government oversight.

But the license was issued for only 15 months — less than the five years typical for taxi licenses. When that license was up in September, the city authorities gave Uber just two months to continue operating in the city and urged reforms.

This month, Uber announced a series of new security measures, including enhanced safety training for drivers, a “discrimination button” that allows drivers and passengers to report abuse, and a direct line to emergency services.

Sadiq Khan, the mayor of London, defended Monday’s decision, saying Transport for London could not be confident that Uber had enough regulation in place to “prevent another serious safety breach in the future.”

“I know this decision may be unpopular with Uber users, but their safety is the paramount concern,” he said in a statement.

Organizations representing drivers of London’s traditional cabs see Uber as undercutting their business and have lobbied hard against the company’s license renewal.

Some, including Steve McNamara, the general secretary of the Licensed Taxi Drivers’ Association, celebrated the decision and said Londoners would be “safer as a result.”

“Unfit operators cannot get away with deliberately shirking their responsibilities,” he said in a statement. “Uber have had 17 months to comply with the conditions of their temporary license, and yet they have continually put Londoners at risk by letting drivers on the road who aren’t properly licensed or insured.”

Uber arrived in London in 2012, and around 45,000 drivers work for the company in the city.

James Farrar, the chair of the United Private Hire Drivers branch of the Independent Workers Union of Great Britain union, said the decision would be a setback for “the most vulnerable work force in London.”

“The mayor’s decision to once again deny Uber a license will come as a hammer blow to its 50,000 drivers working under precarious conditions,” Mr. Farrar said. “Many will now face the distress of facing not only unemployment, but also crippling debt as they struggle to meet car lease payments.”

He called for a meeting with the mayor to discuss “what mitigation plan can now be put in place to protect Uber drivers.”