Theakstone takes on the role of managing director of Group M Trading, which will combine the billings of MindShare, Mediaedge:cia, BJK&E, Mediaedge:cia Manchester and Media Insight.
Based on 2003 Nielsen Media Research billings figures, Group M has combined billings in excess of £900 million. Its main rival is Interpublic Group's Magna, which bills just under the £900 million mark.
Although WPP's negotiation will now become a centralised function, the implementational buying will remain in their respective agencies. Theakstone will also continue in his role as the investment director at MindShare.
Group M will have a three-person board made up of Theakstone, Kelly Clark, the chief executive of MindShare UK, and the chairman of Mediaedge:cia, Rob Norman. It will seek to leverage its billings across all media, not just TV.
Clark said: "For TV deals, the big initiatives will be for 2005, but for the other media we should soon start seeing some impact."
Irwin Gotlieb, the chief executive of Group M Worldwide, said: "Media sales points continue to consolidate in the UK. Group M puts its and our clients in a strong trading position in the UK, and Nick Theakstone is definitely the right man to lead this charge."
Theakstone joined MindShare in 2002 as the head of investment. He was previously the deputy managing director of MediaVest.