Two drivers successfully argued at a tribunal in 2016 that the Silicon Valley firm exerted significant control over them to provide an on-demand service and that they should cease to be considered self-employed, which gives few protections in law.
Uber, which could be valued at $120 billion in a flotation, has faced protests, regulator crackdowns and licence losses around the world as it challenges existing competitors and rapidly expands.
In Britain, the self-employed are entitled to only basic protections such as health and safety but workers receive the minimum wage, paid holidays and rest breaks. Uber has introduced a number of benefits for drivers in recent months.
Unions argue that the gig economy - where people often work for various firms at the same time without fixed contracts - is exploitative, whilst Uber says its drivers enjoy the flexibility and on average earn much more than the minimum wage.
(Reporting by Costas Pitas; Editing by Alistair Smout)
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