A unit of Assured Guaranty Ltd. sued affiliates of Deutsche Bank AG over $312 million of mortgage- backed securities that the bond insurer guaranteed and says were “plagued by rampant fraud and misrepresentations.”

Assured Guaranty Corp. is asking a judge to force the bank to repurchase the loans, on which the insurer has already paid almost $60 million in loss claims and sees the potential for tens of millions of dollars more, according to a complaint filed today in New York state Supreme Court against DB Structured Products Inc. and ACE Securities Corp. The bond insurer, backed by billionaire Wilbur Ross, is also seeking reimbursement for the claims paid and for future losses.

“The entire pools of loans that Deutsche Bank securitized (and to a large degree originated) in the transactions are plagued by rampant fraud and misrepresentations and an abdication of sound origination and underwriting practices,” Assured said in the complaint.

Repurchases of home loans from buyers and insurers of mortgage securities originated before U.S. housing prices began to tumble in 2007 have already cost the four biggest U.S. lenders $9.8 billion, according to Credit Suisse Group AG.

Assured said more than 83 percent of 1,306 defaulted loans examined in one of the transactions, ACE’s Home Equity Loan Trust, Series 2007-SL2, breached Deutsche Bank’s representations and warranties. In the second deal, Home Equity Loan Trust, Series 2007-SL3, 86 percent of the 1,774 loans breached the agreements, Assured said.

AAA Credit Grade

John Gallagher, a spokesman for Deutsche Bank in New York, declined to comment.

Assured was today stripped of its AAA credit grade from Standard & Poor’s, after the New York-based ranking company said investors and issuers are weaning themselves from the guarantees the company offers.

S&P cut its financial strength and counterparty credit rating on Assured Guaranty Corp. and Assured Guaranty Municipal Corp. one level to AA+, according to a statement from the ranking company.

The case is Assured Guaranty Corp. v. DB Structured Products Inc., 651824/2010, New York state Supreme Court (Manhattan).

To contact the reporters on this story: Shannon D. Harrington in New York at sharrington6@bloomberg.net; Karen Freifeld in New York at kfreifeld@bloomberg.net

To contact the editors responsible for this story: Alan Goldstein at agoldstein5@bloomberg.net; David Rovella at drovella@bloomberg.net