Rangers has been accused of conspiring to illegally fix the prices of club merchandise with JD Sports and Elite Sports.
The two clothing companies allegedly fixed the prices of a number of Rangers-branded replica kits and other clothes products from September 2018 until at least July 2019.
The football club also took part in the alleged collusion, fixing the retail price of adult replica short-sleeved home shirts from September 2018 to at least mid-November 2018, the UK's competition watchdog found.
All three parties are alleged to have colluded to ensure the price of the kit was not cheaper at JD Sports than in Elite's Gers Online store, the Competition and Markets Authority said.
Elite was the manufacturer of Rangers-Branded clothing and JD Sports was the only UK-wide retailer selling club clothing at the time.
The CMA said Rangers became concerned at the start of the 2018-19 football season when JD Sports was selling the Rangers replica top at a lower price than Elite, which was seen at the time as the club's so-called retail partner.
They then allegedly conspired with JD to increase the price by nearly 10%, from £55 to £60.
Also, Elite and JD - this time without involvement from Rangers - worked together to fix the prices of Rangers-branded clothing - including training wear and replica kit - over a longer period, the CMA believes.
The watchdog warned the three that they will face fines if they are found to have broken competition law.
Michael Grenfell, from the CMA, said: "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."
Rangers said it would review the provisional findings before entering a response, adding it "treats this matter very seriously".
The club said in a statement: "Rangers notes that it has cooperated with the CMA since the investigation was initiated and will continue to do so."
JD Sports Fashion said it plans to book a hit of around £2m to cover any fine from the CMA and legal costs in its annual results for the year to 29 January, which it will release "shortly".
JD and Elite confessed to 'cartel activity'
Companies found to have breached competition law can be fined up to 10% of their annual turnover.
However, the companies could receive reduced fines if JD Sports and Elite continue to co-operate with the investigation.
The CMA also said that both brands confessed to cartel activity.
Elite Sports has been approached for comment by Sky News.