HPI will become the country's largest independent research agency as a result of the deal, under which ownership will be divided between HPI's 16 partners.
The company fell under the IPG umbrella in January 2000 when it became part of the Draft Worldwide operation.
But David Iddiols, HPI's senior partner, said: "There's been a lack of integration and synergy between us as a market research company and the communications empire that is IPG."
Managers at HPI felt the recent merger of Draft and the IPG-owned FCB had forced the company into an even more peripheral role.
At the same time, HPI, whose clients include Vodafone, Nokia, Kellogg, Virgin and Mercedes-Benz, was concerned that the company's neutrality was becoming increasingly compromised through its link with a number of major agency networks.