JD Sports, Elite Sports and Rangers Football Club have been accused of breaking the law by fixing prices of football shirts, and could face huge fines.
The Competition and Markets Authority (CMA) watchdog thinks the companies colluded to drive up the prices of Rangers clothing at fans' expense.
If this is proven, the firms face fines, and JD Sports has set aside £2million for this.
The CMA said sportswear brands JD and Elite might have fixed the prices of Rangers replica kits and other clothes from September 2018 until at least July 2019.
The competition watchdog said Rangers may also have taken part by fixing prices of short-sleeve adult replica shirts from September 2018 until November 2018.
All three companies have been accused of working together so that JD did not beat football shirt prices on Elite's website, according to CMA provisional findings.
Rangers is a Glasgow-based football team with strong support in Scotland and Northern Ireland.
In 2018 Elite was the manufacturer of Rangers-branded clothing and also sold Rangers-branded products directly through Gers Online Store and later in physical stores in Glasgow and Belfast.
The only UK-wide major retailer selling those products at the time was JD Sports.
The CMA alleges that Rangers became worried that JD Sports was selling the Rangers replica top at a lower price than Elite when the 2018/19 football season kicked off.
It argues that this led to an understanding that JD Sports would hike prices for the Rangers adult short-sleeved home replica shirt by nearly 10%, from £55 to £60, to bring it in line with the prices being charged by Elite.
The CMA is also concerned that Elite and JD Sports – without involvement from Rangers –colluded to fix the retail prices of Rangers-branded clothing, including training wear and replica kit, over a longer period.
This may have led to the firms protecting "profit margins at the expense of fans", the CMA said.
Elite and JD Sports applied for leniency during the CMA’s investigation and confessed to cartel activity.
They face fines if they are proven to have broken the law, but will get a reduction if they co-operate with the CMA.
The CMA can fine firms up to 10% of their yearly worldwide turnover if they break the Competition Act 1998.
CMA executive director of enforcement Michael Grenfell said: "We don’t hesitate to take action when we have concerns that companies may be working together to keep costs up.
"Football fans are well-known for their loyalty towards their teams. We are concerned that, in this case, Elite, JD Sports and, to some extent, Rangers, may have colluded to keep prices high, so that the two retailers could pocket more money for themselves at the expense of fans."
A JD Sports statement said: "The CMA's findings are, at this stage, only provisional and the group will now review them with its advisers.
"The CMA will consider any representations that are made before issuing its final findings.
"As the CMA has noted, JD has co-operated fully with the CMA and, provided this continues, JD will receive a reduction on any financial penalties that the CMA may decide to impose."
A Rangers statement said: "It is not a finding that Rangers has broken the law and does not mean that the CMA will issue a final decision or impose a fine on Rangers.
"Rangers is committed to operating its business in full compliance with all laws, including competition law, and treats this matter very seriously.
"As such, Rangers will review in detail the CMA’s preliminary findings and will be submitting its response to the CMA in due course. Rangers notes that it has cooperated with the CMA since the investigation was initiated and will continue to do so."
Elite has been approached for comment.
