Partners at Freshfields and German firm Deringer Tessin Herrmann & Sedemund have voted for a merger.
The union will create a 267-partner firm to come into effect in January 2000, five months earlier than expected. The firms have been in a loose alliance since January 1998.
Freshfields' senior partner Anthony Salz says: "After a period of very successful integration, Deringer partners have decided to trigger the merger earlier than expected."
Deringer is Germany's ninth largest law firm with 114 lawyers, 32 of whom are partners. It has offices in Cologne, Frankfurt and Berlin as well as Brussels and Moscow. The merged firm will be known as Freshfields Deringer in those offices and Freshfields everywhere else.
Freshfields is the first magic circle firm to accept full merger with a German firm.