Alain Prost says Renault came "very, very close" to quitting F1 altogether in November before deciding to go ahead with its takeover of Lotus.
Renault will return to the grid as a manufacturer team after purchasing Lotus, though the deal remained uncertain for much of the season. Four-time world champion Prost retains close links with Renault and was involved in the discussions ahead of the Abu Dhabi Grand Prix, which he admits were not promising at the time.
"In Abu Dhabi a few days ahead of the weekend, it was almost a no go," Prost told Motorsport.com. "Only [Renault CEO] Carlos Ghosn made the decision. He had to make it himself, but yes, it was very, very close. I haven't talked to him in the last weeks. He knew what to do, and he had to decide himself."
Renault's journey from an engine supplier back to a fully-fledged manufacturer has roots in its fall-out with Red Bull this year, which stemmed from the company's uncompetitive and unreliable power unit. An upgrade at the end of the season failed to show the desired step in performance, meaning Renault finished 2015 still a clear third in the pecking order behind Mercedes and Ferrari.
Prost thinks three years is "the minimum" amount of time it will take for Renault to be challenging for the championship.
"It will take a lot of time to be competitive again. [Ghosn] was talking about three years and I think that is the minimum. It is going to be very hard. But my philosophy is that it is a new project, it's a new story and what is important is that we are going to start in a very difficult environment.
"The deal has been signed a few hours ago. So the first year it is going to be difficult. But what is more important is to build a strategy and to build the story for the future, and to make a progression. That is the most important. You don't need to win next year, it is a long project."
Renault last competed as a constructor between 2002 and 2011, before it became the Lotus team in 2012.