Toyota said twice Formula One world champion Alonso "will explore new motorsport adventures within the Toyota Gazoo Racing family".
No details were given but the Spaniard recently tested a Toyota Dakar Rally car in South Africa. The next edition of that race will be held in Saudi Arabia in January 2020.
Alonso won Le Mans with Toyota last year and will be trying to win the Indianapolis 500 with McLaren later this month to become only the second driver to complete the so-called "Triple Crown of Motorsport."
"Now is a good time to take on new challenges and there are many interesting opportunities to explore within Toyota Gazoo Racing," said the 37-year-old, who left Formula One last year.
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"We still have two important races remaining as we are still in the fight to become WEC World Champions and I will be pushing hard to win both of them."
Hartley won Le Mans with Porsche in 2017 and is a two-time World Endurance champion. He raced in Formula One with Toro Rosso last year.
"I believe I am returning to WEC an even stronger driver following my experience in Formula One," he said.
The 2019-20 WEC season starts at Silverstone on Sept 1 and runs to Le Mans on June 14, 2020.
(Reporting by Alan Baldwin; Editing by Ken Ferris)
