As big banks scramble to form rescue plans for bond insurers, a hedge fund manger is trying to send the ambulances away.

Bill Ackman, whose Pershing Square Capital Management has been betting on a collapse of the bond insurers for several years, has written to regulators, urging them not to support these efforts.

In a letter to Federal Reserve and Treasury officials obtained by Portfolio.com, Ackman criticizes the bank bailout efforts that have been prompted by regulators, saying that this strategy "appears to be aimed at propping up insolvent and falsely rated entities so that banks can defer judgment day to a time when they are better capitalized or their stock prices are higher."

The fear is that the bond insurers will not be able to meet their guarantees amid a wave of defaults. That could result in credit-rating downgrades or collapse, leaving the banks on the hook for tens of billions of dollars in losses on their holdings of collateralized debt obligations, or C.D.O.'s.

"While we understand that the banking industry counterparties to the bond insurers would prefer to avoid taking these C.D.O. risks back on balance sheets—particularly at a time when their balance sheets are strained by subprime and other losses that have not been hedged, there are no such free lunches in the capital markets," Ackman writes in the February 5 letter.

Ackman says that banks and bond insurers should be forced to disclose their C.D.O. exposures in detail, and that federal regulators should work with state officials to ensure that bond insurers "preserve as much of their capital as possible for the benefit of policyholders and bank counterparties."

The Wall Street Journal reports that consortiums of banks are trying to find ways to unwind the credit-default swaps that they took out with the bond insurers as ways to guarantee the banks' C.D.O.'s.

Banks would receive stakes in the bond insurers, Financial Guaranty Insurance and Ambac Financial, in exchange, the Journal says.

Bankers are afraid that a collapse of bond insurers could have painful repercussions throughout global credit markets.

"It could be a tsunami-like event comparable to subprime," Josef Ackermann, the chief executive of Deutsch Bank, said in an interview with Bloomberg Television.