The prospect of a Publicis joint trading arm in the UK was last raised in 2006 before the Publicis Groupe Media chairman, Jack Klues, effectively ruled it out due to lack of any "definitive client benefit".
Discussions on joint trading between the agencies will be headed by Linda Smith, the Starcom MediaVest UK chief executive, and Gerry Boyle, the chief executive of ZenithOptimedia.
The shape of the unit and its management structure have yet to be decided and clients will be consulted as part of the process.
The new model is unlikely to be as sweeping as WPP's Group M media parent structure, with Publicis sources indicating that the two agency brands would continue to take the lead.
However, the agencies have decided to explore the advantages that their combined £1.2 billion buying power could deliver in negotiations with media owners.
Starcom MediaVest is the fourth-ranked UK media business with £658 million in billings, while ZenithOptimedia is sixth with £565 million in billings, according to Nielsen Media Research.
Steve King, the chief executive, worldwide, of ZenithOptimedia, said: "While creating the new PGM unit to deliver to BT, it would be naïve not to explore what benefits could be achieved in exploring future synergies between ZenithOptimedia and StarcomMediaVest Group in the UK."