NEW YORK (Bloomberg) — Insurers cut their prices on corporate board coverage by an average 18 percent, the steepest decline in three years, a Towers Perrin survey of US buyers showed.An annual index generated by the survey fell 183 points to 827, the lowest level in five years, the Stamford, Connecticut-based consultant said in a report yesterday. Had the index been limited to repeat participants from the past two surveys, the decline would have been 6.5 percent, Towers Perrin said. Public companies paid an average premium of $801,497.

Board insurance protects companies and officials who may be sued by shareholders for negligence or misleading statements. Insurers such as American International Group Inc., the largest board insurer by premium volume, and No. 2 Chubb Corp. have been cutting prices as the perception of risk from scandals such as Enron Corp. in 2001 fades with new accountability standards.

A survey by Advisen Ltd., a New York-based consultant, shows a slower decline in rates since a peak in 2003. In January the company said rates had fallen only 16 percent from the peak and in the fourth quarter of 2006, were an average 4.4 percent less than the year before.