"Uini Atonio was admitted to La Rochelle hospital yesterday following suspected heart problems," the Top 14 side said in a statement. "Medical tests confirmed a cardiac incident. His condition is now stable and he remains under observation in intensive care."
"Following his hospitalisation, Uini will need to undergo a long period of convalescence. It is now clear that he will not be able to continue his playing career."
The 35-year-old prop had been initially called up last week by France coach Fabien Galthie to prepare the Six Nations opener against Ireland. However, the French Rugby Federation announced on Monday that Atonio was out for the match, without specifying the reason.
Atonio, who earned 68 caps with Les Bleus, won the Six Nations title in 2022, including the Grand Slam, and in 2025.
The New Zealand-born prop was instrumental in La Rochelle's back-to-back Champions Cup triumphs in 2022 and 2023, further cementing his legacy at the club he has been playing for since 2011.
He last appeared for La Rochelle in a Champions Cup fixture against Harlequins on January 18.