Prosecutors at a federal labor agency have determined that Amazon is a joint employer of subcontracted drivers who delivered packages for the company in California, pushing back on claims from the online retailer that they are not its employees.

The decision, made by a regional director for the National Labor Relations Board in Los Angeles, came after the agency investigated unfair labor practice charges filed against the company by the Teamsters union.

The prominent labor group represents UPS drivers and has been seeking to unionize Amazon drivers. However, it has encountered challenges, most notably because the company doesn't directly employ drivers but relies on thousands of third-party businesses that deliver millions of customer packages every day.

Currently, over 275,000 drivers are employed with these businesses, which are called Delivery Service Partners, or DSPs.

The Teamsters and other labor advocates have long said Amazon exercises great control over the drivers - including by determining their routes, setting delivery targets and monitoring their performances - and should be classified as a joint employer.

Last year, the Teamsters said they unionized dozens of drivers who work for one DSP in Palmdale, California called Battle Tested Strategies. The labor group filed several unfair labor practice charges against Amazon after the company refused to negotiate a union contract with them.

On Thursday, NLRB spokesperson Kayla Blado said agency prosecutors made "merit determinations" on three of those charges, one being that Amazon and Battle Tested Strategies were joint employers of the drivers who work for the firm.

Prosecutors also determined Amazon made unlawful threats and failed to provide relevant information to the union. They further found that the two employers "unlawfully failed and refused to bargain with the union over the effects of the decision to terminate" the DSP's contract last year, Blado said.

However, the prosecutors dismissed other charges against Amazon, she said, including one that alleged that the company's decision to terminate its contract with the unionized DSP was a retaliatory move.

If a settlement is not reached, the agency can bring a complaint against Amazon, which would be litigated within the NLRB's administrative law system. Amazon has the option to appeal a judge's order to the agency's board and eventually, to a federal court.

"As we have said all along, there is no merit to the Teamsters' claims," Amazon spokesperson Eileen Hards said in a statement. "If and when the agency decides it wants to litigate the remaining allegations, we expect they will be dismissed as well."

Meanwhile, Teamster General President Sean M. O'Brien championed the findings.

"Amazon drivers have taken their future into their own hands and won a monumental determination that makes clear Amazon has a legal obligation to bargain with its drivers over their working conditions," O'Brien said.