The Financial Services Authority yesterday warned Britain's listed football clubs that more information about player transfers and other market-sensitive information ought to be announced through the Stock Exchange.
Ken Rushton, director of listing at the FSA, made it clear in a letter sent to the nine listed clubs that the watchdog was unhappy with the amount of speculation on websites or in newspaper back pages without companies making formal announcements to City.
The move came after the FSA looked into the rumours that preceded David Beckham's transfer in June from Manchester United to Real Madrid and other moves during the period of summer transfers.
The FSA decided not to take any action over Beckham's departure, about which the club made a formal announcement to the Stock Exchange. But the letter said clubs should release any price-sensitive information as quickly as possible.
An FSA spokesperson said: "Football clubs are subject to a level of media scrutiny which is quite different from other listed companies. So when they are talking to fans or the press they need to remember they are answerable to shareholders as well."
Manchester United said it "followed the FSA's guidelines".
However, Peter Ridsdale, ex-chairman of Leeds United, suggested the FSA's move would have a limited effect. "It is right and proper to use the Stock Exchange as the first port of call, but in my experience transfers rarely have an impact on share prices. When I bought Rio Ferdinand for £18m and sold him for £30m there was not much of a change," Mr Ridsdale said.
Mr Rushton wrote: "The UK Listing Authority recognises that at certain times of the year, particularly during open transfer windows, the likelihood of price sensitive developments increases, as does press speculation in relation to such developments."
The letter is separate from the FSA's inquiry into the takeover of Chelsea. The FSA launched an investigation late last month into shareholdings in Chelsea in the run-up to the London club's takeover by the Russian billionaire Roman Abramovich, saying publicly disclosed shareholdings may have been inaccurate and the stock market could have been misled about the "true ownership" of the club.
Shares in the club's parent company, Chelsea Village, jumped 30 per cent in the two weeks leading to the announcement on 1 July.The nine clubs the FSA wrote to were Aston Villa, Leeds United, Manchester United, Newcastle United, Southampton, Sunderland, Tottenham Hotspur, Celtic and Heart of Midlothian.