Red Bull driver Mark Webber has decided to leave Formula 1 at the end of this season.
The 36-year-old Australian is joining Porsche's new sportscar programme and will compete in the classic Le Mans 24 Hours race for the German manufacturer.
Webber had options to stay in F1, either with Red Bull or another top team, but feels it is time to move on.
Lotus driver Kimi Raikkonen, the 2007 world champion, is favourite to replace Webber, who made his F1 debut in 2002.
Toro Rosso's Daniel Ricciardo and Jean-Eric Vergne are also options.
Webber described it as an "honour" to race for Porsche, saying it "has written racing history as a manufacturer and stands for outstanding performance and technology at the highest level".
He added: "I'm very much looking forward to this new challenge after my time in Formula 1. I can hardly wait to pilot one of the fastest sports cars in the world."
In 12 years in F1, Webber, who also drove for Minardi, Jaguar and Williams, has won nine grands prix, achieved 36 podium finishes, 11 pole positions, 15 fastest laps and twice finished third in the World Championship.
His best season was 2010, when he led the championship for much of the year before faltering in the final three races, falling behind team-mate Sebastian Vettel and Ferrari's Fernando Alonso.
With a reputation as a tough but fair competitor, plus a direct and honest approach to the media, Webber's time at Red Bull has been characterised by an uneasy relationship with Vettel.
The tension between the two drivers has boiled over on several occasions during their five years as team-mates, most recently at this season's Malaysian Grand Prix, when Vettel ignored a direct team order not to overtake Webber to win the race.
The German initially apologised after the race, but subsequently said he would do the same thing again.
Webber, who has often felt he has not benefited from the same support that Vettel enjoys from Red Bull motorsport adviser Helmut Marko, was furious, prompting talk that he would leave the team at the end of 2013.
But Red Bull boss Dietrich Mateschitz, with whom Webber has always had a close relationship, made it clear he could stay on if he wanted to.
"Whether he retires, only he will choose, but Mark is definitely a candidate for 2014," said Mateschitz in April. "He is always welcome with us."
Webber considered staying with Red Bull for another season, but, with a big change in regulations next year that will dramatically alter the demands of F1, he has decided to seek a fresh challenge.
He has also been critical of the Pirelli tyres that F1 teams have been forced to use since 2011, claiming their rapidly degrading nature prevents drivers pushing to the limit throughout a race.
Webber is 37 this August, an age that, for a Formula 1 driver, he has admitted is "long in the tooth in some people's eyes". He also recognised that, even if he stayed on for another year, his time in the sport was running out.
Webber could have gone to another top team - at least one approached him - but he felt that would be the wrong thing to do.
For one thing, Red Bull has produced the fastest car in F1 since 2009. For another, he decided against a move to Ferrari last year partly because he felt that the effort of getting used to a new environment was not worth it given of his limited time left in the sport.
Webber will now spearhead Porsche's attempt to revive its glory days at Le Mans, a race it has won 16 times, the last in 1998.
He has raced at Le Mans twice before - with Mercedes in 1998 and 1999.
He was on pole position in 1998 only for the car he shared with Germans Klaus Ludwig and Bernd Schneider to retire after just 19 laps.
The following year, an aerodynamic flaw in the Mercedes car caused him to flip on two separate occasions at more than 200mph.
On both occasions, he escaped injury, but said he would never return to Le Mans because of the danger involved.
Webber will also compete in the World Endurance Championship for Porsche.
"I'm very pleased to have secured Mark Webber for our LMP1 project as one of the best and most successful Formula 1 pilots of our time," said Wolfgang Hatz, Porsche board member for research and development.
"Mark is without doubt one of the world's best race drivers. He has experience at the Le Mans 24 hour race and, on top of that, he's been a Porsche enthusiast for many years."