The offices of JJB and Sports Direct were raided on Thursday as part of an investigation over possible price fixing and fraud.

The Office of Fair Trading is in the early stages of a probe of the sportswear retailers, triggered after JJB blew the whistle on suspected cartel activity in January.

The Serious Fraud Office also confirmed on Thursday that it was pursuing a parallel criminal investigation after a referral from the OFT.

JJB said: “The directors take seriously JJB’s obligations under UK competition law and, since approaching the OFT, the company has worked closely, and continues to co-operate fully, with the OFT and all aspects of its investigation.”

JJB also said that it will have immunity from any fine of up to 10 per cent of turnover or any criminal prosecution resulting from price-fixing charges if they arise. It is not clear whether that immunity also applies to any potential fraud charges.

JJB would not comment further on what the activities under investigation were, but people familiar with the matter said that the OFT was looking at the sale of 31 stores from JJB to Sports Direct.

A Competition Commission probe has already ruled that the sale raised antitrust issues, resulting in a ruling that Sports Direct should sell five of those stores.

But one person close to the situation said the current probe was far wider-reaching than focusing on only those deals.

Sports Direct said the investigation referred to “a suspected over-arching agreement to dampen competition in the sports retail market”.

Sports Direct said it would co-operate with the investigation.

JJB’s shares fell 10.3 per cent on the news while Sports Direct dropped 16.3 per cent.

JJB said the OFT was looking into activities during a period when former chief executive Chris Ronnie was at the company between June 2007 and March 2009.

Mr Ronnie denied any malpractice relating to the OFT and SFO probes. He said: “I am disappointed that I have been singled out in JJB’s announcement. I have never taken part in any anti-competitive behaviour.” His lawyer said he was co-operating with the probe.

Mr Ronnie is a long-time friend and ally of Mike Ashley, the owner of Sports Direct and Newcastle United Football Club.