(Recasts throughout to include PartnerRe charge, Channel Re background, "no comment" from RenRe, byline)

By Lilla Zuill

NEW YORK, Jan 16 (Reuters) - Bermuda reinsurers RenaissanceRe Holdings Ltd (RNR.N: Quote, Profile, Research) and PartnerRe Ltd (PRE.N: Quote, Profile, Research) said on Wednesday they will write off 3-year-old investments in Channel Re, a reinsurer formed solely to do business with MBIA Inc (MBI.N: Quote, Profile, Research), the world's largest bond insurer.

The announcement comes after Channel Re notified the companies that fourth-quarter losses stemming from its business with MBIA are expected to exceed its shareholders equity.

RenRe said its investment in Channel Re carried a value of $126.7 million at the end of September. PartnerRe said it would take a $74 million fourth-quarter charge, equal to about $1.31 per share, to write down its investment.

Reinsurers effectively insure other insurers, spreading the risk of losses among more than one party.

Channel Re expects $200 million in credit impairments from a $3.3 billion mark-to-market charge at MBIA, according to Partner Re's statement.

David Lilly, an outside spokesman for RenRe, declined to comment further on the Channel Re development. PartnerRe could not immediately be reached for comment.

Rising defaults in mortgage-related bonds have threatened to wipe out a significant amount of capital for bond insurers such as MBIA, putting ratings under threat of downgrade.