Hayes and Stephens will take Manning Gottlieb's £15 million Virgin Mobile strategic planning business and the £1 million Ann Summers planning business with them as launch clients.
The agency, as yet unnamed, will launch in January, offering communications planning for clients. Hayes, Manning Gottlieb's planning director, and Stephens, its marketing and new-business director, are in talks with OMD Europe about the network taking a minority stake in the start-up.
Hayes and Stephens, who also runs part of Manning Gottlieb's Virgin business, are launching the agency after Virgin Mobile came to them with a proposal to maximise creativity in its planning.
Stephens said: "This gives us an opportunity to build upon our achievements at Manning Gottlieb and allows us the freedom to create, source and develop exciting ideas across the industry."
Hayes and Stephens both previously worked at Saatchi & Saatchi. Stephens joined Manning Gottlieb five years ago and Hayes in 2001.
Their departures are said to be unrelated to Manning Gottlieb's recent restructure and creation of a central planning unit called The Source, headed by Neil Hurman, a former ad director at Mirror Group Newspapers.
Nick Manning, the chief executive of Manning Gottlieb, said: "Andrew and Ben wanted to take advantage of a special opportunity. Given the relationship between us all, it was obvious we could help each other. They are very talented individuals and I am sure they will be successful."