By Heather Somerville and Dan Levine:

The U.S. Chamber of Commerce sued the city of Seattle on Thursday over an ordinance that allows drivers of ride-hailing apps Uber [UBER.UL] and Lyft to unionize, saying it violates federal antitrust laws.
 Seattle last year became the first U.S. city to pass a law giving drivers for companies such as Uber and Lyft, as well as taxi and for-hire drivers, the right to collectively negotiate on pay and working conditions.
 City officials took action amid growing concerns about how drivers are compensated. Both Uber and Lyft vigorously opposed the measure, arguing that existing federal labor law trumps local legislation.