Havas Sports UK will support EDF, the sustainability partner of London 2012, by recruiting brand ambassadors and coordinating a range of marketing activities to publicise EDF's sustainability credentials.
Promotional activities will include a "Carbon challenge" campaign, designed to encourage 12m people across Europe to commit to cutting their carbon footprint by 15% before 2012.
Keith Impey, chief executive of Havas Sports, said: "Sponsorship of London 2012 gives EDF a powerful platform to take ownership of the sustainability debate and establish clear leadership on this hugely important issue.
"We're fortunate enough to have a healthy existing relationship with EDF, but the Olympics represents a different challenge entirely, and we had to show we could handle an event of such unique size and importance.
"Our work supporting the London 2012 bid and our longstanding involvement with the IOC, dating back to 1992, were key factors in us clinching the deal."
Havas Sports UK negotiated EDF's sponsorship of the Rugby World Cup this year and the sports agency has also assisted EDF with the Anglo-Welsh Cup and sponsorship of Arsenal FC.
Louise Poole, EDF Energy's head of brand and UK project director for London 2012, said: "We've been consistently impressed with Havas Sports UK's support of our brand and its unique understanding of our business objectives.
"Havas Sports UK's track-record of Olympic involvement, most notably its support of our partnership of the London 2012 bid in 2004, makes them extremely hard to ignore when it comes to handling such a magnificent opportunity as sponsorship of the Olympics."
Havas Sports UK is part of the Havas Media Group's global sports marketing division. Its clients include Sky Sports, Totesport, Yorkshire and Clydesdale Bank, Tourism Malaysia, Fullers, Friends Provident, ASDA, Air Asia and F&C Investments.