The warning caused Marconi's share price to tumble, and began the firm's fall from grace resulting in a debt-for-equity swap which will leave shareholders with almost nothing.

Trading in the company's shares had been suspended for the day before the profit warning.

The FSA said Marconi broke market rules by failing to immediately release information which was price sensitive.

Tough decision

The inquiry criticised the company's former management team, including the then chairman Sir Roger Hurn and chief executive Lord Simpson, for delaying the release of the warning by 24 hours.

"In this case a major listed company failed to provide important information to investors on a timely basis," said FSA managing director Carol Sergeant.

"We require companies to keep the market informed of price sensitive information without delay so that investors can be sure they are making financial decisions based on the most up to date information."

Sir Roger and Lord Simpson issued a joint statement which noted that the FSA said they had not acted in "bad faith".

"Whilst we are clearly disappointed at the conclusion the FSA has drawn, the regulations require directors to make difficult judgments," the statement said.

"In our case we were acutely conscious of our duties to the market, including the duty only to make a statement once satisfied that it was based on reliable information."