The Hanover Group has entered into an agreement to buy renewal rights to parts of a book of business owned by OneBeacon Insurance Group.

The agreement applies to small commercial and middle-market policies, with the majority written by agents that already have a relationship with Hanover, according to A.M. Best Co., a ratings group.

The deal affects policies written starting Jan. 1, 2010.

The transaction did not affect A.M. Best’s ratings for Hanover or its property-casualty insurance members. The ratings service predicted the transaction will have a modest impact on premium leverage, and A.M. Best anticipates the overall operational impact also will be modest given Hanover’s experience with previous acquisitions.