Spain's Alonso won the world title in 2005 and 2006
Renault's two-time world champion Fernando Alonso believes Formula 1 is "finished" following the announcement of plans for a breakaway series.
On Thursday, eight of F1's major teams, including Renault, said they intended to establish a rival championship.
And Alonso told the BBC that what was left of the old F1 would be inferior.
"Formula 1 is finished. It will be a standard engine, small teams and no drivers - this is not the Formula 1 people want," the Spaniard said.
"The new series will be the new Formula 1."
The breakaway move is the latest chapter in the bitter row over budget cap proposals.
World motorsport boss Max Mosley wants to introduce a voluntary £40m budget cap to curtail a "financial arms race" among F1's teams.
But eight of them, under the umbrella of the F1 Teams Association (Fota), have strongly resisted Mosley's attempts to force this and other rule changes through.
Mosley's latest letter proposes a £86m budget cap for 2010, falling to £39m in 2011.
Ferrari, Toro Rosso and Red Bull have refused to race under the optional £40m cap originally proposed, with only Williams and Force India definitely on the starting grid next season.
Speaking before the teams issued their breakaway threat, Alonso said: "I hope they find a solution, but these are difficult times. It is a critical position for Formula 1.
"If we do not race in Formula 1 next year it changes many things. It won't be the same. We know we'll race, but it will be different."
Asked if he would drive in a different series next year if the worst case scenario does unfold and there is a split, Alonso replied: "Of course. I won't retire, I will drive for another championship.
"We want to compete with the best teams in the world, the maximum technology, we all want to compete with the best drivers.
"If there is no new series, I will not join the new Formula 1 as I have said, with the small teams.
"It is not F1, not the category we have loved for the last 60 years.
"There are other categories, and hopefully I'll find an attractive one outside of F1, or outside Europe. I will keep racing because I am only 27 years old.
"Maybe the US. There is some good competition there, like the Indy cars, and then there is Le Mans. I am not going to close any doors."