Legal action is being taken on behalf of couriers who claim they are being denied their rights because they are classed as self-employed.
The GMB union has launched proceedings at an employment tribunal on behalf of eight workers at Hermes.
It is the latest case of workers in the so-called gig economy saying they are denied rights such as holiday pay and the national living wage because they are classed as self-employed.
Maria Ludkin, the GMB's legal director, said:Â "GMB will fight bogus self-employment and exploitative practices whenever and wherever we can.
"Under the false claims of 'flexibility', Hermes seems to think it's acceptable to wriggle out of treating its workers with respect.
"Guaranteed hours, sick pay and pension contributions aren't privileges to be bestowed when companies feel like it, they are the legal right of all UK workers."
Michael Newman, of law firm Leigh Day, which is taking the case for the union, said:Â "We believe that Hermes are deliberately avoiding giving their couriers the rights to which they are entitled.
"They do so by labelling the couriers who work for them as self-employed, when the reality is different.
"We have started employment tribunal proceedings in order to challenge this, so that these couriers can enforce their rights as workers."