The 37-year-old has confirmed he will leave McLaren and not race in the sport in 2019. The Spaniard is now expected to make a full time switch to America's Indy Car series.
Alonso won the Drivers' Championship in 2005 and 2006 while with Renault, ending the five-year dominance of Michael Schumacher and Ferrari in the process. However, despite still being considered one of the best drivers on the F1 circuit, he has not won an F1 race in five years.
"After 17 wonderful years in this amazing sport, it's time for me to make a change and move on," said Alonso.
"I want to thank everyone at McLaren. My heart is with the team forever. I have built so many great relationships with many fantastic people at McLaren, and they have given me the opportunity to broaden my horizons and race in other categories."
Alonso is now 20/1 with Sky Bet to win an F1 race before the end of the season, having nine Grand Prix left to compete in.
Tripe crown challenge awaits
Alonso is into this 17th season in F1, his fifth with McLaren, and has amassed 32 wins, 22 pole positions and 97 podiums to date. His victory in 2005 made him the then-youngest world champion in F1 history.
While leaving the door open on a possible return to F1 in the future by stating there could be a right moment for him to return to McLaren when they "come back stronger and better", he will now focus on completing motorsport's 'triple crown'.
With two Monaco Grand Prix victories under his belt and now June's Le Mans 24 Hours success, at his first attempt, Alonso just needs to win the Indianapolis 500 to equal Graham Hill's achievement.
Alonso said: "Let's see what the future brings; new exciting challenges are around the corner. I'm having one of the happiest times ever in my life but I need to go on exploring new adventures."
McLaren have not yet said who will replace Alonso.
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