Law firm Leigh Day, with the support of campaign group 38 degrees, is bringing an employment tribunal case to test the legality of the firm's treatment of part-time employees.
All part-time staff at the retailer are said to be on the zero-hours contracts, meaning they are not guaranteed any hours or income each week.
However, Kathryn Dooks employment lawyer at Kemp Little has told HR magazine that European case law means it will be hard to bring a claim of discrimination against Sports Direct.
"Past European case law states that individuals working under zero-hours contracts could not compare themselves with full-time employees and so could not bring a claim of discrimination on the grounds of being a part-time worker, (which requires the part-time worker to point to a comparable full-time employee when showing less favourable treatment)" she said.
"The European Court of Justice found that zero-hours staff were not comparable with full-time colleagues precisely because full time colleagues were required to work a fixed number of hours each week for a fixed salary and did not have the option of refusing work (unlike individuals on zero-hours contracts)."
No flexibility
Former Sports Direct employee and 38 Degrees member, Zahera Gabriel-Abraham, 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 Gabriel-Abraham, said: "We are not arguing that employers cannot have genuine flexible contracts, but the contract under which Gabriel-Abraham worked, and which all Sports Direct 20,000 part-time employees appear to be working, has no flexibility at all for those people who sign them.
"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."
Earlier this week a study from the CIPD found more than one million workers in the UK are on zero-hours contracts.
