Trainer sellers JD Sports and Footasylum have been fined £5m after their chief executives unlawfully discussed commercially sensitive information without telling the mergers regulator.

The Competition and Markets Authority (CMA) said Peter Cowgill of JD Sports and Barry Bown of Footasylum held two undisclosed meetings last summer as the former sought to secure a takeover of the latter.

They had been ordered to keep their companies separate while the CMA decided whether to block the merger to protect consumers from the threat of reduced competition and higher prices. The £90m deal was ultimately barred in September.

The regulator said that when Mr Cowgill and Mr Bown met they discussed Footasylum’s financial performance, its stock allocations from key brands and the closure of stores, which was forbidden.

They were criticised for their failure to keep notes of their meetings, at least one of which took place in a car park, and for allegedly destroying some of what records were created.

JD Sports and Footasylum were fined a total of £4.7m. The penalty is the second largest of its type levied by the CMA, after Facebook was told to pay £50m in November after it breached a similar order following its purchase of Giphy.

JD Sports was fined £4.3m and Footasylum £380,000.

Kip Meek, chair of the inquiry group investigating the merger, said: “There is a black hole when it comes to the meetings held between Footasylum and JD Sports. Both CEOs cannot recall crucial details about these meetings.

“On top of this, neither CEO nor JD Sports’ general counsel can provide any documentation around the meetings – no notes, no agendas, no emails and poor phone records, some of which were deleted before they could be given to the CMA.”

JD Sports said that it had always acted “honestly and in good faith” and there were no rules to prevent chief executives from meeting.

In November, video footage emerged of Mr Cowgill meeting Mr Brown in a car park near Bury in Greater Manchester. The material was shared with the CMA by a third party.

The FTSE 100 company also accepted that it “inadvertently” received limited commercially sensitive information, and that some phone records were not available.

However, the retailer accused the regulator of publishing incorrect or misleading information, and of using “inflammatory language” in relation to some of its conclusions. It denied deleting phone records.

It is understood that the CMA’s decision to fine JD Sports and Footasylum is backed by hundreds of pages of evidence, which will be published in the coming weeks.

The decision comes after the deal between the two sportswear companies collapsed, with the CMA demanding in November that JD Sports offload the retailer.

Mr Meek of the CMA added: “This fine should act as a warning – if you break the rules there will be serious consequences.”