Dewey Ballantine and LeBoeuf, Lamb, Greene & MacRae have agreed to merge, creating a firm of more than 1,300 lawyers.

Expected to become effective by October, subject to partner approval, the planned combination is the largest ever between New York-based firms. The combined firm, which will be known as Dewey & LeBoeuf, will have 550 lawyers in New York, making it the fifth-largest in the city.