A legal challenge is being brought against retail giant Sports Direct over the use of zero hours contracts among its staff.
Law firm Leigh Day, with the support of campaigns group 38 Degrees, is bringing an employment tribunal case to test the legality of the treatment of the firm's part-time employees.
The move follows news that all part-time staff at Sports Direct are on zero hours contracts.
38 Degrees member, Zahera Gabriel-Abraham, 30, from London, has brought the legal claim funded by other 38 Degrees members through donations.
Elizabeth George, a barrister in the employment team of Leigh Day, who is acting for Ms Gabriel-Abraham, said: "We are not arguing that employers cannot have genuine flexible contracts, but the contract under which Ms Gabriel-Abraham worked, and which all SportsDirect.com 20,000 part-time employees appear to be working, has no flexibility at all for those people who sign them.
"There was no practical difference between the obligations put on my client by the company and those placed on full-time staff.
"Casual workers traditionally supplement an employer's salaried staff, to be called upon when cover is needed or demand is high. In return for not having the security of knowing when you might work you have the benefit of being able to choose when you work. Without that choice you are not a casual worker you are just a worker with no job security.
"The 'casual' part-time employees in this case are employees in the conventional sense and denying them their paid holidays, sick pay and bonuses is unlawful."
David Babbs, executive director of 38 Degrees said: "We've had a huge response from our members on this issue, many of whom are on so called zero hours contracts themselves. These contracts clearly work in certain circumstances, but in others, are being abused by employers to restrict people's rights.
"That's why we've asked our members to help fund this legal action - so we can send a clear message to SportsDirect.com that thousands of people are standing with Zahera in her legal claim."
A study this week suggested that a million workers were on zero hours contracts, under which people are not guaranteed work from one week to the next.