The announcement coincided with an announcement by Roche of plans to separate the roles of chairman and chief executive, both of which are held by Humer. Roche said the move, seen as a surprise, was "in view of the increasing complexity of the tasks involved".

When he assumes the role, Humer, a non-executive director of Diageo for the past two years, will be, in effect, chairman of two businesses that would both have FTSE 100 status were they to be listed in the same country.

Theoretically, that violates the Higgs code on corporate governance, which says no individual should be chairman of more than one FTSE 100 company, although it also gives businesses the opportunity to "comply or explain".

A spokeswoman for Diageo said the drinks group had conducted an internal and external search before making the appointment and "was not aware this would be an issue".

Humer joined the Swiss drugs company 12 years ago as head of its pharmaceuticals division. He moved there from Britain's Glaxo after losing out in a struggle for the top job.

Humer had worked there for 14 years and was chief operating officer before his move to Switzerland. He had previously spent 14 years at the US drugs company ScheringPlough, in Britain, Portugal and Ecuador.

Roche said it was appointing Severin Schwan, the littleknown Austrian who is the head of the company's diagnostics division, as chief executive.

The shake-up comes against a background of attempts in Switzerland to improve corporate governance and a recognition of the danger of concentrating too much power in the hands of one person.

Blyth said: "The board is delighted that Franz Humer will assume the role of chairman next year. Franz has a first-class track record with considerable experience of international business in both mature and developing markets."

Humer said: "I am delighted and honoured that my board colleagues have offered me this appointment. Over the past two years, I have come to know and admire Diageo's people and performance. I look forward to taking up broader responsibilities."

Diageo's brands include Guinness, Johnnie Walker Red Label and Smirnoff Vodka.