The culture of fear in Sports Direct’s warehouse has yet to be eradicated, workers claimed, as the company promised on Tuesday to improve conditions as part of a number of pledges published in an internal report.

Sources close to the retailer’s depot in Shirebrook, Derbyshire, said they continue to be afraid for their jobs and were too scared to raise grievances.

One worker employed in Shirebrook through an employment agency – like the majority of staff at the warehouse – said that during the summer some employees had been switched to rotas in which they had to work extra shifts at short notice, creating problems with childcare.

The source said: “They were changing rotas at very short notice. You must go [to work the shift] because people are scared. If you say you can’t go to work the agency then calls you to say ‘sorry, we don’t need you’.”

Two other sources indicated that workers remained worried that complaining about the treatment of workers inside the facility could cost them their jobs.

The comments were made as Sports Direct published an internal investigation into working conditions that was conducted by the retailer’s law firm, RPC.

The report received a mix reception from trade unions and politicians who had been pressuring the group to improve its widely criticised working practices. There were concessions on zero-hours contracts for shop workers and a backtracking on the controversial six-strikes-and-you’re-out policy in the warehouse. However, the report stopped short of pledging to employ about 4,000 warehouse workers directly via Sports Direct rather than on temporary agency contracts that currently apply to the vast majority of warehouse workers.

One source close to the warehouse operation said the group did not have the internal human resources staff qualified to manage 4,000 full-time warehouse workers, even if it wanted to.

In its report, Sports Direct pledged to bolster its human resources operation to ensure there was “no culture of fear”. The report was published in the wake of the Guardian’s investigation into the retailer last year, when undercover reporters exposed how the group was paying many warehouse workers less than the national minimum wage.

The Guardian investigation also spoke to local primary school teachers who said that parents who worked in the warehouse were sending their childrento school while ill because they were concerned they would lose their jobs if they took time off to care for a sick child.

After sustained efforts to avoid giving evidence to a subsequent parliamentary inquiry into working conditions at the retailer, billionaire founder Mike Ashley finally appeared in June after being warned that he could have been found in contempt of parliament if he refused. At the hearing he confirmed the Guardian’s reporting by admitting that the company had broken the law by paying some workers less than the legal minimum.

HM Revenue & Customs, along with the union Unite, which has led a long-running campaign highlighting conditions in Shirebrook, forced the company into making back payments of wages to short-changed minimum wage workers. The bill is thought likely to approach £1m for the retailer and its employment agencies, who are also thought to be facing fines totalling about £2m.

RPC’s report said that Sports Direct’s failure to pay warehouse workers the legal minimum was “unintentional”.