Aug. 24 (Bloomberg) -- John Browne, former chief executive officer of BP Plc, left Apax Partners Worldwide LLP to help expand the energy business of rival buyout firm Riverstone Holdings LLC.

He will be a managing director in a new London office of New York-based Riverstone, the firm said today in a statement. Apax said Browne's resignation as chairman of the advisory board is effective next month and no immediate replacement will be made.

Browne, 59, who built London-based BP into what was once Europe's biggest company through more than $100 billion of acquisitions, quit on May 1. Riverstone will use his 41 years of experience at BP to identify alternative- and renewable-energy markets after oil prices more than doubled in four years.

``His role at Apax was more as an adviser, whereas he will have a more of an executive position at Riverstone,'' said David Currie, head of a unit which manages about 3.7 billion euros ($5.1 billion) at Standard Life Investments in Edinburgh. ``It must be quite a catch for Riverstone to gain someone with his experience in the energy sector.''

Browne was CEO for his last 12 years at BP, the world's third-largest oil refiner. He resigned after failing to block the Mail on Sunday newspaper from publishing claims that he used BP's resources to help support a former boyfriend. The London High Court said Browne lied under oath about details of the affair. BP said the allegations that he misused resources were unfounded.

Ronald Cohen's Support

He joined Apax's advisory board in 1999 and became chairman last November. The group oversees the firm's five main investment areas: technology and telecommunications, retail and consumer goods, media, health care and financial and business services.

Apax founder Ronald Cohen was one of 67 business leaders to publicly back Browne in a letter published in the Financial Times. Browne left the board of directors of Goldman Sachs Group Inc., the biggest securities firm, a week after he left BP.

``This is an opportunity for me to return to the energy industry full-time, which will enable me to best apply my life- long skills and experience,'' Browne said in today's statement.

To contact the reporter on this story: Jon Menon in London Jmenon1@bloomberg.net

Last Updated: August 24, 2007 11:06 EDT