The watchdog is looking into a ninefold jump in call option trading volumes at HCA in the days before the American hospital chain announced a $33 billion (£18 billion) takeover by a private equity consortium including Kohlberg Kravis Roberts (KKR).

Call options confer the right to buy shares at a specified price in the future and their purchase implies an expectation that the stock price will rise.

The SEC investigation, which is at a preliminary stage, has fuelled fears that the surge in “club” private equity deals could spark an outbreak of insider trading.

It comes only a month after the watchdog froze an estimated $862,000 in profits that it said could have been earned from insider trading in Petco Animal Supplies. Suspicious trades occurred a few days before the American pet products company was sold for $1.62 billion to the buyout firms Leonard Green & Partners and Texas Pacific.

Regulatory experts say that the surge in multibillion-dollar buyouts, which routinely involve three or four partners and can comprise as many as seven private equity firms, has greatly increased the number of lawyers, bankers, secretaries and other advisers participating in increasingly complex transactions.

Marc Powers, a former SEC enforcement lawyer in New York, said: “The more people involved in a transaction, the more important it is to ensure that all participants understand their obligations not to tip people off.”

Jon Najarian, a trader in Chicago with a website devoted to spotting unusual trading patterns, said that the HCA trades looked “pretty suspicious because no important industry news was being announced and other hospital stocks weren’t moving”.

“Cases of private-equity related insider trading are likely to increase because, as deals get bigger, there are now so many more people involved in the transactions who could pass on the information,” Mr Najarian said. “I would say things are already starting to heat up. In the last three weeks I have seen a number of unusual actions.”

Private-equity related insider trading concerns have also been fuelled by Justin Huscher, a former managing director of Madison Dearborn Partners, the American buyout firm. Without admitting or denying guilt, Mr Huscher, who left the firm in 2004, last month agreed to pay more than $108,000 in so-called disgorgement and civil penalties.

The SEC said that Mr Huscher had made more than $54,000 in illegal profits by buying Unisource Energy shares in 2003 after he learnt of its impending acquisition from a friend who was involved in the consortium that bought the power company.

The SEC declined to comment on the HCA investigation.

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