US regulators have underestimated potential bank losses on preferred stock issued by Fannie Mae (NYSE: FNM - News) and Freddie Mac, the American Bankers Association said on Monday.

Nearly a third of US banks hold preferred stock issued by the two mortgage financiers that were taken into conservatorship this month, according to an industry survey conducted by the ABA. The average bank exposure to such securities relative to core equity capital was 11 per cent.

"The negative impact on banks - particularly Main Street community banks - is far greater than regulators first thought," wrote Edward Yingling, chief executive of the ABA in a letter to the Treasury, the Federal Reserve and other banking regulators.

The government takeover of Fannie and Freddie all but wiped out the value of $36bn of their preferred shares. This would force exposed banks to take writedowns at the end of the third quarter that could impede future lending, the ABA warned.

"When the actions were contemplated to reduce dividends on Fannie Mae and Freddie Mac preferred stock, the bank regulators estimated that only a dozen banks would be affected by it," Mr Yingling said.

Regulators said this month only a small handful of banks had "significant" holdings in Fannie Mae and Freddie Mac relative to their capital bases and that they would help develop plans to restore capital at these banks.

However, the ABA survey suggests the impact of writedowns could be more widespread and more severe than regulators initially indicated, particularly among small community banks that engage in lending for small and medium-sized local businesses.

The ABA's letter called for regulators to reconsider the suspension of dividend payments on Fannie and Freddie preferred stock to alleviate the capital impact on banks and avoid a multi-billion dollar decline in lending.