Fernando Alonso is unlikely to move to Renault next season, after the team's managing director admitted their car might not be competitive enough for the Spaniard.

Alonso, whose contract expires in 2018, has previously indicated he wants to be in a competitive car in 2018 adding he would make a decision on his future by September. The 36-year-old, who was forced to retire in the Belgian Grand Prix on Sunday (27 August), has picked up a meagre 10 points so far this season and has not finished any higher than fifth since returning to McLaren in 2015.

Still widely considered one of the best drivers in the sport, Alonso has made no mystery of his desire to be in a top team next year, but Renault managing director Cyril Abiteboul fears his team might still fall short of his expectations.

The French team has steadily improved since returning to F1 as a constructor in 2016 has picked up 34 points in the first 12 rounds of the season but Abiteboul insisted a return to Renault would be premature for Alonso, who won two world titles in his first spell with the team.

"Things have to be compatible so that it can be a successful association, and not just because it has been a successful association in the past," he told Autosport.

"It's the future that we're worried about. He has his dynamic, I think he has urgency to be in a position to be fighting for championships again. We know that it's going to take us a bit of time to have a car that can offer that, so clearly the one thing that I would not want is to have a frustrated Fernando in a Renault car, that's for sure."

With Nico Hulkenberg already confirmed for next season, a number of drivers have been linked with Renault's second seat, which is currently occupied by Jolyon Palmer. Force India duo Sergio Perez and Esteban Ocon have both been mentioned as potential candidates, as have Carlos Sainz jr and Robert Kubica, who returned to the sport earlier this month.

The Pole, who suffered career-changing arm injuries in a rally accident six years ago, completed the equivalent of two race distances of the Hungarian GP in Renault's 2017 ahead of F1's summer break.

Abiteboul admitted Renault needed more answers over Kubica's prospects to return to F1 but the sport's limits on testing meant it was unlikely the situation would be resolved anytime soon.

"If we had no restrictions from the regulations there is a lot that we could do, by organising more testing, but it's exactly the same for a number of young drivers to try to make it into F1," he explained. "There is a restriction in place, and it's not going to change for Robert. So we will have to see if the timing of what we have to do with him can work with the timing of the rest of the paddock, which is moving."

However, despite the uncertainty surrounding his future, the Renault managing director insisted Kubica was not a "plan B".

"I will never call Robert a plan B or plan C," he said. "The main question is can he race again in F1? If the answer was a straightforward 'yes', you would probably not be asking me that question. But unfortunately it is not clear yet."