For the second time this month, a federal judge has rejected a challenge to Seattle's first-in-the-nation law allowing drivers of ride-hailing companies such as Uber and Lyft to unionize over pay and working conditions.

U.S. District Judge Robert Lasnik late Thursday rejected a challenge brought by 11 drivers. He earlier rejected a challenge brought by the U.S. Chamber of Commerce. The organization is appealing that decision. The National Right to Work Legal Defense Foundation, which represents the drivers, said Friday that it would appeal too.

But the judge declined to keep Seattle's law on hold pending the appeals, clearing the way for the drivers to unionize unless the 9th U.S. Circuit Court of Appeals says otherwise.

The 2015 law requires companies that hire or contract with drivers of taxis, for-hire transportation companies and app-based services to bargain with them if a majority show they want to be represented.

Uber said in a statement that the ruling was not surprising, but would negatively impact thousands of drivers.

"The original ordinance passed by the City Council was never about benefiting drivers, but about helping Teamsters and taxi companies," said Uber General Manager for the Pacific Northwest Brooke Steger. "We will continue fighting to protect independent drivers and prevent turning back the clock on transportation in Seattle."