Fernando Alonso might have blown his chance of ever winning a third title by switching to McLaren-Honda this year.
That is the claim of 1997 world champion, who said Spaniard Alonso’s "early years at Ferrari" was the best driving he has ever seen in formula one.
But he lost patience with the Italian team last year, and is now trailing the field with the burgeoning McLaren-Honda project.
"Leaving Ferrari was clearly a mistake," French Canadian Villeneuve told the Spanish sports daily AS.
"He could have spent three or four more years with them and then retired or gone to McLaren if he wanted to win with them as he did in 2007," he said.
Alonso has defended his move on the basis that, in order to beat Mercedes in the new turbo V6 era, he needed to help McLaren-Honda build up a new project from the ground.
The 33-year-old has also said he doubts Ferrari - although resurgent this year - is ready to win the 2015 title.
But Villeneuve said: "I think Alonso would have fought for the title with this Ferrari."
Alonso, however, says he was happy to take a ’risk’ with Honda rather than settle for what could have been more near-successes in a Ferrari.
"With this move," Villeneuve insisted, "Alonso may have ended his career — he may not win any more titles or even races."
Villeneuve, who raced in F1 with works Honda power for years, says he has little faith the Japanese manufacturer can quickly rise to the front of the grid.
"I worked with them," he said, "and know it can be a very slow culture and one that prevents them from doing certain things."