McLaren's racing director Eric Boullier says a decision on their engine partner for next season needs to be made by September.

McLaren, who have won the Constructors' Championship eight times, have been in partnership with Honda since 2015 although the two-and-a-half seasons that have followed in Formula 1 have been testing for both parties.

The Woking-based outfit have been hit by numerous reliability issues while Fernando Alonso has not held back when criticising the team for being unable to produce a car capable of completing a grand prix.

Results have slightly improved this year - although the team are still sitting ninth in the team standings - and reports suggest McLaren are eager to keep Honda as their engine provider for the 2018 campaign.

However, the Japanese supplier is believed to be weighing up a deal with Toro Rosso while rumours also claim the company could decide to leave the sport completely.

Boullier admits a decision needs to be made by the end of September.

"Technically you could change an engine in 12 weeks," he told Autosport. "You can change it anytime before the season, up to three months before. If you want to do a base design around an engine, it's September."