France prop Uini Atonio has had to retire from all rugby with immediate effect after being hospitalised on Tuesday "for a heart attack", his club La Rochelle announced in a statement.
The 35-year-old New Zealand-born star had been a pivotal figure for France under coach Fabien Galthie, winning two Six Nations titles, including the Grand Slam in 2022.
He was still part of Galthie's plans, having been named in his initial squad last week for the upcoming defence of their Six Nations crown.
"Atonio was admitted to La Rochelle hospital yesterday with a suspected cardiac problem," read the statement from the Top 14 club.
"The medical exams confirmed it was a heart attack. He is stable but he will remain, under surveillance, in intensive care."
Atonio, who was capped 68 times by France and won two Champions Cup trophies with his club, had pulled out of La Rochelle's match with Clermont on Sunday after suffering chest pain during the warm-up.