Shares in Aon plunged, falling nearly 25% in morning trade, down $5.37 at $15.87, with the blow worsened by news that it had only broken even in the second quarter.

Aon said the Securities and Exchange Commission (SEC) had questioned several items on its accounts and now wants the company to release more information in its financial reports in the future.

Aon has stressed that the SEC's questions have focused more on presentation of its accounts, rather than any actual accountancy problems. The SEC may however rule that the company has to restate its earnings for the past three years.

The fraud at telecoms giant WorldCom and a wave of similar accounting scandals has heightened investor concern about corporate accounting practices.

"We firmly believe that any adjustments made will not affect shareholder equity," Aon chief executive Patrick Ryan said. "I believe they ( the SEC) are going to try to help us resolve these issues, so that we won't have any concerns."

No spin-off

However shareholders were also struck by the fall in the last quarters' earnings. The company broke even, compared with a profit of 11 cents a share in the same period last year.

The results were hit by a one-off charge of $36m, but even excluding such one-off items, the results were sharply lower than last year.

While the earnings were a "major disappointment", the company forecasts a better performance in the second half of the year.

Share in Aon have already fallen 55% this year.

The company has also dropped plans to spin off its underwriting unit and is now considering selling Combined Specialty Group instead.

"An awful lot of people owned Aon for the spin off," Bill Batcheller, who helps manage $89bn at National City Corp and owns Aon shares.

"They hoped to benefit from the upside in the insurance brokerage business once the underwriting unit was gone."

Headquartered in Chicago, Aon employs 53,000 people.