A subsidiary of the Marsh insurance brokerage is buying one of the largest regional insurance brokerages in New England, an acquisition that will make Marsh & McLennan Agency the country’s 12th largest insurance agency.

In a move it says “dramatically accelerates its national growth,” Marsh & McClennan Agency has acquired Roanoke, Va.-based Thomas Rutherfoord Inc., one of the nation’s largest independently owned firms.

Following the acquisition of one of the Southwest’s largest independent agencies, Marsh & McLennan is expanding its national presence with one of the Northeast’s biggest firms.