The move will create the UK's eighth-largest agency with billings of £221 million, based on Campaign's most recent Top 300 Agencies report.
It will take the name Euro RSCG Wnek Gosper Partners despite the impending departure of Brett Gosper, the co-chairman, for McCann-Erickson in New York.
Its formation spells the end of the Arnold Group, the collective unit of agencies within Havas that Partners BDDH operated with WCRS and EHS Brann.
The Partners BDDH chief executive, Nigel Long, will become the chief executive of the new agency, while Euro's Adam Leigh and Partners' Simon Toaldo will become the joint managing directors. The agency's creative department will be run by Partners' executive creative director, Nick Hastings.
The new agency will become the advertising unit of Havas' recently created group, Euro RSCG Partners. The group's chairman, Mark Wnek, and the chief executive, Chris Pinnington, will take co-chairman roles at the new ad agency.
In addition, Wnek will continue to work as a creative partner, collaborating with Hastings on specific accounts, such as Peugeot.
Partners' marketing director, Neil Christie, will take the same position at the new agency. Euro's equivalent, Christian Hinchcliffe, will handle new business at a group level.
The merger will reunite the Peugeot and Citroen car accounts under one agency. They were separated at the start of 2002, when Citroen was moved out of Euro and into Partners as a result of creative differences. Wnek and Long are adamant that both car clients are happy about the merger.
But the situation is less clear for Euro's Del Monte and Air France accounts, which clash with Partners' Gerber Foods and bmi accounts.
The merger will result in redundancies, although the management team has yet to determine which sectors of the agency these will affect.
Pinnington said: "There will be a small number of redundancies, but both agencies are profitable and well-run, so the numbers will be limited."
On the new management line-up, Long said: "We've just spent the past three weeks getting the roles right. This is not a fudge. It's important that people's positions are as clear in the new agency as they were in the old one."
Wnek added: "Euro RSCG Wnek Gosper Partners is an agency that can now compete on all bases with any agency in the UK."
The move will not affect the DM agency Partners Andrews Aldridge, which is half-owned by Partners BDDH.