App-based minicab service Uber is facing legal action over claims it is failing to provide basic rights to its drivers.
The GMB union will challenge the company's claim that its workers are partners rather than employees.
The union says Uber is breaching its duty on pay, holidays, and health and safety.
The firm says making drivers employees would mean losing their flexibility, which makes the job appealing.
'Substantial pay outs'
GMB, the union which represents professional drivers, has instructed the law firm Leigh Day to take action on behalf of members driving for Uber.
Nigel Mackay from the firm believes legal action could result in "substantial pay outs" for drivers: "We believe that it's clear from the way Uber operates that it owes the same responsibilities towards its drivers as any other employer does to its workers," he said.
"In particular, its drivers should not be denied the right to minimum wage and paid leave."
The GMB's Steve Garelick said: "Operators like Uber must understand that they have an ethical and social policy that matches society's expectations of fair and honest treatment."
Uber describes itself as a "pick-up" service that connects those needing a ride with a background-checked private driver, and takes a cut - typically 20% - of the fee.
In May, Transport for London reported the number of private hire vehicle licences had risen from 52,000 to 77,000 over the previous 12 months, and most of that increase down to new Uber drivers.
Legal action in the UK comes just weeks after a California court ruled a driver for Uber was an employee, not a contractor. Uber insisted the ruling only applied to the individual driver making the claim and said it would appeal the ruling. Classifying Uber drivers as employees would severely dent Uber's profits and likely lower its $40bn (£25.62bn) valuation.
Nigel Mackay went on yo say, it was time Uber took "responsibility" for its drivers and treated them as any other employer does.
"A successful legal action against Uber could see substantial pay outs for drivers, including compensation for past failures by the company to make appropriate payments to who we argue are their workers," he said