No ratings actions were taken on the firm's insurance company, Combined Insurance Company of America. The Insurer Financial Strength (IFS) has a "AA" rating or "F1-plus" for short-term financial strength rating. The IFS remains on Rating Watch Negative, the rating agency said.

Fitch said it made the rating change to reflect the firm's "high financial leverage position relative to expectations," and the cost of its business transformation plan, which includes the spin-off of Combined into a separate company.

In order to make up for the loss in revenue from the divestiture of Combined, the brokers insurance services and consulting business will be required to "perform at high levels to maintain expected cash flow coverage ratios."

However, Fitch said Aon's fundamental core business remains strong and "merits strong ratings for both its senior debt and its commercial paper."

A spokesman for Aon would not comment on the report.

As of 11:30 a.m., Aon's stock was down 28 cents to $34.98 a share.

Completion of the spin-off of Combined is expected during the second quarter of this year.