The Leo Group and Bartle Bogle Hegarty have agreed to merge their respective UK media operations, Starcom and Motive, to form a new company, the Starcom Motive Partnership.
The merger, which has been known about for some time, will create the UK's third-largest media agency with estimated billings of more than £400 million.
The announcement comes as the Leo Group and the MacManus Group press ahead with their merger, unveiled last year, which will create the world's fourth-largest advertising network.
As part of that merger Starcom and MacManus's MediaVest will merge in some markets. But where the two are both established, as in the UK, a merger is unlikely.
The Starcom Motive Partnership will be headed up by motive managing director, Mark Cranmer, who will take on the same role at the merged agency.
Cranmer will report to Jack Klues and Bob Brennan, chief executive officer and chief operating officer respectively of Starcom Worldwide in Chicago.