Rangers investor and Sports Direct founder Mike Ashley has been accused of a cynical publicity stunt after it emerged thousands of workers would miss out on an offer of guaranteed hours.

Directly-employed casual staff will get the option to work at least 12 hours a week, instead of their current zero-hours contract.

But 4,000 agency workers will lose out, despite making up the bulk of employees at Sports Direct's warehouse in Shirebrook, Derbyshire.

Critics said that thousands would still start each week unsure how many hours they would work or how much they would earn.

Sports Direct also said it would also suspend its "six strikes and you're out" disciplinary procedure and pay warehouse staff above the National Minimum Wage.

The U-turns follow an investigation by Mr Ashley’s own lawyers, who found "serious shortcomings" at Shirebrook.

The Sports Direct board said that it "deeply regrets and apologises for" the problems.

The Unite union has accused the retailer of subjecting staff to Victorian working practices, in which they lived in constant fear of losing their job or being disciplined for "excessive" talking or spending too long in the toilet.

Workers likened conditions to a gulag, according to the union.

One woman even gave birth in the toilet, while others complained that long security searches at the end of their shift meant that they took home less than the minimum wage.

The company and its controversial owner were accused of offering "too little too late" in a bid to head of a showdown with some of its biggest shareholders.

Critics questioned why many staff would miss out on guaranteed hours.

Scottish Labour economy spokesman Jackie Baillie said the announcement looked like “a millionaire searching for good PR instead of a boss who will give workers a fair deal”.

"It is not good enough to simply offer guaranteed hours to some staff but not all staff.”

Neil Gray, the SNP’s fair work spokesman, said: “There is far more to do to ensure all workers are treated fairly and equally.”

TUC general secretary Frances O'Grady said: "An apology is always a good start, but this is too little, too late. What we really need is an independent investigation."

Unite called on the board to reconsider its proposal to move just 10 agency workers a month onto direct, permanent contracts.

The union said that at that rate it would take 28 years to affect the entire workforce.

Mr Ashley has faced increasing pressure from shareholder groups in the run-up to this week’s annual general meeting (AGM).

There have been calls for changes to the company’s board of directors and for an immediate independent review into working conditions at its factories.

Despite the reforms, the billionaire is still expected to face shareholder and union anger at the meeting, where he will open the retailer's factory doors to the public as part of an “open day”.

Jon Trickett, Labour’s shadow business secretary, welcomed what he said were “good steps forward”.

But he added: “I hope this isn't the last word from Mr Ashley, and that we can look forward to further such announcements in coming weeks, particularly regarding warehouse staff.”

He added that workers could not rely on “bad publicity” to stamp out every instance of employers mistreating their staff and called for a ban on zero-hours contracts.

Sports Direct also announced that it would employ a nurse and welfare officer at Shirebrook after unions found a large number of ambulance callouts and pledged to carry out fewer searches.

Since the controversy emerged, Sports Direct has vowed to pay thousands of warehouse workers £1 million in back pay.