Mike Ashley's Sports Direct has been accused of reneging on a promise to offer guaranteed hours to staff on zero hour contracts in a fresh blow to the company ahead of its annual meeting.
Unite the union claims the troubled retailer has been looking to take on casual staff on zero hour contracts, despite vowing to stop the practice during last year's AGM.
Sports Direct said it would offer guaranteed hours to all staff following a review by its law firm RPC, which it launched in response to claims that "Victorian" style working conditions were rife throughout the firm.
However, Britain's biggest union said the retailer was still advertising for causal workers and offering flexible contracts on the Sports Direct store fronts and jobs website.
It comes as Unite and investment manager Hermes added their names to the chorus of dissenting voices calling for chairman Keith Hellawell to step down following the AGM at the firm's controversial warehouse in Shirebrook.
Steve Turner, Unite's assistant general secretary, said: "This revelation shows it is 'business as usual' at Sports Direct and casts doubt on just how sincere it is about cleaning up its act.
"Made amid great fanfare, Sports Direct's commitment to wean itself off exploitative zero hours contracts and offer store staff guaranteed hours was meant to demonstrate the retailer was serious about dealing with abusive working practices.
"Yet one year on Sports Direct has been caught red handed breaking its promise to offer workers the security of knowing what hours they will work and how much they will earn from one week to the next.
"With the retailer advertising for causal workers in its Sports Direct and upmarket Flannels stores across the UK, it is clear this is no mistake, but a return to the bad old ways once the spotlight had gone away.
"It blows a hole in Sports Direct's commitment to treat workers with dignity and respect."
Unite has said it will stage protests outside the AGM and at Sport Direct's flagship Oxford Street store in London.
Pressure has been mounting on Mr Hellawell after a number of investors in the retailer publicly voiced their opposition to his re-election ahead of a crunch vote on Wednesday.
The former West Yorkshire Police chief constable and government drugs tsar has presided over a long list of scandals and controversies at the tracksuit chain, and has pledged to step down if he does not receive the support of a majority of independent shareholders at the firm's AGM this week.
In January, Mr Hellawell saw 54% of independent shareholders vote against his reappointment and was only saved by Mr Ashley, who owns 55% of the company and whose backing meant he was re-elected with a total of 80.92% of votes cast.
Leon Kamhi, head of responsibility at Hermes Investment Management, welcomed the return of Mr Ashley as chief executive of the company, but said it was the right time for the chairman to step aside.
He said: "It is our view that it is now the appropriate time for Keith Hellawell, chairman, and Simon Bentley, senior independent director, to step down from their roles at Sports Direct.
"Therefore, consistent with the 2016 AGM, we have recommended votes against both of their positions."
Royal London Asset Management and Standard Life Aberdeen joined a trio of shareholder advisory firms - Institutional Shareholder Services, PIRC and Glass Lewis - who came out in opposition to Mr Hellawell being reappointed.
But the under fire chairman will find solace in the fact that Phoenix Asset Management Partners and Aurora Investment Trust intend to vote for his reappointment.
Mr Ashley will not be in attendance at the AGM because of "conflicting demands".
Last year the tycoon was hauled before MPs to be grilled over "Victorian" working conditions at the retailer's warehouse, the company hosted a farcical "open day" at its headquarters, and its chief executive Dave Forsey quit - only to be replaced by Mr Ashley.
The Newcastle United owner admitted paying some staff below the minimum wage and was taken to task over the firm's controversial use of zero-hour contracts.
In the meantime, profits at Sports Direct have tumbled and shareholders have become restless.