Fresh complaints about "Victorian" working practices at Sports Direct have been made, with conditions likened to a "labour camp".
The Unite union told MPs that workers at the retail giant's Shirebrook warehouse in Derbyshire were in constant fear of losing their job, facing disciplinary action for spending too long in the toilet, or for "excessive" talking.
It was also revealed that HM Revenue and Customs stepped in to investigate claims of underpayment of hundreds of directly-employed workers after Unite said they were not being paid the minimum wage.
Assistant general secretary Steve Turner told the Business Select Committee that workers were exploited, with thousands employed on zero-hours contracts, facing the sack if they had six "strikes" placed against them by managers.
In a submission to the committee, Unite said Sports Direct's business model was built on "cheap, disposable labour", with workers hired through employment agencies on "contradictory" contracts.
Workers likened conditions to a "gulag", or "labour camp" and unless the company takes urgent steps to address "serious malpractice", its reputation will be tarnished and it will be seen as a poster boy for bad business, said Unite.
The union said it had no confidence in a review into working practices announced last year by owner Mike Ashley, claiming that little was happening behind the "gloss of PR stunts".
Thousands of workers were removed from the warehouse during a recent media visit, which saw a general clean-up operation, including doors placed on toilets, it was claimed.
Workers who do not have a bank account have their wages paid via a debit card, which costs them £10, with a monthly management fee of £10, as well as 75p per cash withdrawal. Workers also pay between 45p and £2.45 a week for insurance services.
Unite said it had major concerns over health and safety, claiming workers turned up sick for fear of being sacked.
A total of 110 ambulances or paramedic cars were dispatched to the warehouse over the last two years, with 50 classed as "life threatening", claimed Unite.
Regional officer Luke Primarolo said one woman had given birth in a toilet at the warehouse, telling MPs that workers were "scared".
Chris Birkby, managing director of Transline, which supplies workers to Sports Direct, said HR surgeries had been set up for staff to raise any problems.
Regular surveys are held, with one last week showing a 96% satisfaction level.
"That does not show an unhappy workforce," he said.
Finance director Jennifer Hardy said problems had been dealt with and there had been a "massive improvement " at the warehouse.
They told the committee that the pre-payment cards were voluntary and were aimed at speeding up payment to workers.
Andy Sweeney, chief executive of Best Connection Group, which also supplies staff, accused Unite of "misrepresenting" conditions at Shirebrook.
"It is a modern distribution warehouse where the concerns of workers are catered for. The negative press is misrepresented."
He said that to the best of his knowledge no-one was mistreated.