Ntamack limped out of the World Cup hosts’ warm-up game against Scotland in Saint-Etienne on Saturday.
A statement from the French federation on Monday morning confirmed that the Toulouse playmaker had ruptured his anterior cruciate ligament (ACL), ending his hopes of featuring on home soil this autumn.
Ntamack’s club colleague Cyril Baille, France’s regular starter at loosehead prop, has also emerged as a World Cup doubt.
Baille is set to miss five to six weeks of action with a right calf issue.
Bordeaux-Begles fly half Matthieu Jalibert offers a high class back-up option for France, while Antoine Hastoy, who won the Champions Cup with La Rochelle earlier this year, is the other specialist fly half in Fabien Galthie’s World Cup training squad and likely to make the final 33.
Full back Thomas Ramos and scrum half Antoine Dupont can also cover No 10 if required.
But Ntamack’s injury is a major blow for the hosts ahead of the tournament, with the 24-year-old firmly established as first-choice fly half over the last few years, starting every game of last year’s Six Nations as France secured a grand slam.
Multi-talented prop Baille, meanwhile, has also been a key part of the side’s development into genuine tournament contenders. Jean-Baptiste Gros and Reda Wardi are the other looseheads currently available to Galthie.
France begin their World Cup campaign against New Zealand in an eagerly-anticipated opening night fixture in Paris on 8 September. They also face Italy, Uruguay and Namibia in Pool A.
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