HSBC Holdings Plc has withdrawn from lawsuits challenging bond insurer MBIA Inc's $5 billion (3 billion pounds) restructuring, leaving six banks still in the cases.

Agreements ending HSBC's participation in the lawsuits were filed Monday in New York State Supreme Court in Manhattan. No explanation was included in the filings.

Bank of America Corp , BNP Paribas SA , Morgan Stanley , Natixis SA , Societe Generale and UBS AG remain in the cases.

The banks filed parallel lawsuits in 2009 against the Armonk, New York-based insurer and the state's insurance superintendent, who had approved the split of MBIA's municipal bond business from its structured finance operations.

The banks claim the restructuring was intended to defraud policyholders. They claim it left MBIA undercapitalized and possibly unable to pay their claims.

The remaining clients are fully in this litigation to undo MBIA's massive fraudulent conveyance, said Robert Giuffra, Jr., a partner at law firm Sullivan & Cromwell LLP who represents the banks.

MBIA attorney Marc Kasowitz did not immediately return a call for comment.

Wells Fargo & Co and Royal Bank of Scotland Group Plc , among other banks, also have pulled out of the lawsuits.

The cases are ABN Amro Bank NV, et al, v. Eric Dinallo, 601846/2009, and ABN Amro Bank NV et al v. MBIA Inc., 601475/2009, both in New York state Supreme Court (Manhattan).

(Editing by Gerald E. McCormick)