The chief of one of America's best known organic supermarket chains has been forced to make a humiliating public apology for secretly promoting his company and denigrating its main rival for years using a fake name in Yahoo! online chatrooms.

John Mackey, the chief executive of Whole Foods Market Inc, which owns 196 stores worldwide and has just opened an outlet in Kensington, London, issued a statement today asking "stakeholders to please forgive me" after the US's Security and Exchanges Commission announced it was launching an investigation.

The affair, which saw Mr Mackey admitting to posting on a Yahoo! stockmarket chat forum under the name "Rahodeb," is considered a breach of marketplace rules. The chief executive's anonymous postings talked up his company while predicting a bleak future for its main rival, Wild Oats Markets Inc, while at no point disclosing his identity.

Mr Mackey's postings are particularly prejudicial as his company has announced it is intending to buy Wild Oats, its chief competitor in the health and organic food market, for $670 million (£328m).

"I sincerely apologise to all Whole Foods Market stakeholders for my error in judgment in anonymously participating on online financial message boards," the chief executive said, in a short statement. "I am very sorry and I ask our stakeholders to please forgive me."

Mr Mackey says his postings were made under the name Rahodeb because it is an anagram of his wife Deborah's name. Despite posting for seven years, one particular comment in 2005 attracted the attention of the Securities and Exchange Commission.

In it, Rahodeb speculated that Wild Oats' stock price, then $8 a share, would soon slide below $5, a move which could be considered an attempt to unfairly influence the marketplace.

The error is considered so serious for the Texas-based company's future that business analysts believe Mr Mackey may soon be forced to resign. His company's board of directors has formed a special committee to investigate the affair.

Nell Minow, a corporate governance expert, told the Reuters news agency today that it was a case of "monumental poor judgment" and that Mr Mackey's actions could lead to him being dismissed.

"The fact that somebody has twisted his arm into saying he's sorry just isn't enough," she said.

"When you are the CEO of a public company, you no longer have the luxury of communicating in any way you like about what's going on in your company."

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