Seattle has become the first major US city to pass a law giving drivers for Uber

and Lyft the right to unionize, the latest twist in a heated debate about how workers for app-based ride services should be treated.

The law approved unanimously by the Seattle City Council recognises the right of drivers for on-demand ride companies known as Transportation Network Companies, as well as taxi and other for-hire drivers, to collectively negotiate on pay and working conditions.

Councilperson Mike O'Brien, who proposed the law, said he expects Uber to sue the city to stop the measure. Opponents including Uber claim that federal labor laws preempt such lawmaking by a city.

Supporters say Uber and Lyft drivers aren't addressed by the National Labor Relations Act, the 1935 bill passed to protect employees, giving cities room to act.

The Seattle bill is just one of several legal battles being fought by the ride service across the United States.

Uber operates in more than 300 cities in 67 countries and has raised US$7.4 billion from investors. Its war chest has helped fund legal and regulatory battles across the world, and lobbying efforts at the state and national levels.

Although the bill doesn't address drivers' employment status, it does extend to them rights usually reserved for employees.

At least 1,000 drivers have already organised as part of the App-Based Drivers Association. Many have said they struggle to make a living, with some earning less than US$3 per hour after expenses.

- Reuters