Retail giant Sports Direct faces a claim for millions of pounds from almost 300 workers over zero-hours contracts.
The part-time staff claim that they have been excluded from a £160m ($242m, €211m) bonus scheme, which paid out to 2,000 "permanent" workers in 2013, because they were on the flexible contracts.
Leigh Day, which represents the workers, has been preparing multiple claims for breach of contract against Sports Direct, the Guardian reported.
The solicitors said they have sent letters to the employer's legal team claiming just more than £1m in compensation for missed bonuses for a first lot of 30 workers.
The FTSE 100 firm, which was founded by Newcastle United owner Mike Ashley, came under fire for its use of zero-hours contracts last year.
Labour leader Ed Miliband claimed 17,000 of the companies' workers were on the flexible employment agreements.
"These Victorian practices, the epidemic of zero-hours contracts that we see at Sports Direct, have no place in the 21st Century," said Miliband.
Elsewhere, the business secretary Vince Cable has said that the UK government plans to crackdown on companies that "abuse" zero-hours contracts by banning exclusivity clauses.
The Office for National Statistics (ONS) said that there were 1.4 million zero-hours contracts in the UK in late 2013.
The Chartered Institute of Personnel and Development (CIPD) has argued that the contracts have been unfairly "demonised" and that some employees welcome the flexibility.
A 2013 survey from the HR body found that 65% of zero-hours workers said they are satisfied with their work–life balance compared with 58% of employees not signed up to one of controversial contracts.
IBTimes UK contacted Sports Direct for a statement but the employer declined to comment.