BDML has picked up the 25,000-policy Hill House Hammond (HHH) motorcycle book from Norwich Union (NU), according to market sources.
The business, which includes the Mitchell's portfolio, is estimated to have cost around £1m.
BDML is expected to transfer staff from the old HHH office in Southampton to its Portsmouth headquarters on completion of the deal.
Market sources said that a decision was yet to be made about the Mitchell office in north London.
A market source said: "BDML will either be looking at this deal as a platform to grow its motorcycle business to critical mass or it could be aiming at a quick resale once a 12-month exclusion clause imposed by NU is over."
BDML declined to comment.
The company reported it expected to double its managed policy count by the end of the year, mainly through its deal to take on NU's failed policies and by extending its relationship with RAC.