He is rebranding all of Ogilvy's businesses, such as the direct marketing division OgilvyOne, the design agency Coley Porter Bell and the marketing communications company 141, under the Ogilvy Group UK banner.
A group board, made up of 60 group partners selected from the 11 companies and reporting directly to Leih, will be set up to run the company.
Leih refused to confirm who would sit on the board, but Guy Lambert and Mike Dodds, the joint managing directors at OgilvyOne, will both be included, as will Paul Jackson, the chief executive of Ogilvy & Mather.
However, Jackson's role could be redefined as part of the overhaul.
Leih said the restructure will allow clients to choose between using one of the separate companies or to take advantage of a fully integrated offering.
He said: "Ideally, we will have a centralised planning function which will not be discipline-specific but will contain all of the disciplines. However, clients will still have the freedom to work independently within any of those disciplines.
"Bundling together generally comes around in seven-year cycles and we are now coming into a new one. Many clients are looking for this integrated approach that we will be able to offer and other agencies can't."
A move out of Ogilvy's Canary Wharf offices has also been mooted.