Sports Direct said it will offer directly employed casual retail staff at least 12 guaranteed hours a week, instead of zero-hour contracts. However almost all staff at the firm's troubled Shirebrook warehouse are agency workers and are not eligible.

In a report commissioned by the firm, it apologised for conditions at Shirebrook, which have been likened to those of a Victorian workhouse. The company has been under mounting pressure to overhaul the way it is run.

Sports Direct's offer of guaranteed hours will apply to the firm's 18,250 casual staff who work in its stores. The 4,059 warehouse workers supplied by agency staff, will not qualify for the offer. Just 40 of the firm's warehouse employees are on permanent contracts.

Last year an investigation by the Guardian newspaper revealed that warehouse staff were subject to lengthy security searches which, in some cases, resulted in their pay falling below the legal minimum wage. And a BBC investigation found ambulances were called out to Sports Direct's complex at Shirebrook, in Derbyshire, 76 times in two years.

In Tuesday's report, the firm said its failure to pay some staff at its Shirebrook warehouse the minimum wage was "unacceptable but unintentional", and said it had a new pay policy in place.

It also said it would ask agencies to suspend their "six strike system" for misdemeanours under which staff were given "a strike" for spending too long in the toilet, excessive chatting or taking a day off sick. Once an employee had six strikes they were automatically dismissed.

The report, conducted by law firm Reynolds Porter Chamberlain, found the system "contributed at times to a hierarchical and potentially oppressive model."

Additional changes the firm has promised include:

• Appointing a full time nurse at its Shirebrook warehouse "who will hopefully be in a position to offer professional advice about when an ambulance is or is not required"
• Providing additional training for warehouse supervisors to "ensure there is should be no culture of fear"
• Providing a confidential reporting system for victims of sexual harassment
• Considering a test scheme transferring ten agency staff a month to Sports Direct
• A tannoy policy to ensure it is only used for logistical purposes and not to criticise staff for not working hard enough

The report said Sports Direct owner Mike Ashley "takes ultimate responsibility for any aspects of the working practices that were unsatisfactory".

The company holds its annual meeting for shareholders on Wednesday. Shareholders have called on the firm's billionaire founder, who also owns Newcastle United Football Club, to improve both corporate governance and working practices at the company.

However the firm warned that "only so much can be achieved" in the three months since it started to reform its business and it admitted it will take "far longer to improve the general culture".

In June, in an appearance in front of the Business, Innovation and Skills select committee investigating working conditions at the firm, Mr Ashley admitted the firm had "probably" outgrown his ability to run it.

He said at the time that much of what he'd found out, after starting an internal investigation into how staff were treated at its Shirebrook distribution centre six months ago, was an "unpleasant surprise".

The firm said it had already commissioned a second review of working practices to monitor progress.