LONDON • The Canadian Grand Prix could determine whether Red Bull decide to stick with Renault, who powered them to all their four Formula One championships, or switch to Honda engines from next season.
Team boss Christian Horner said after Sunday's Monaco Grand Prix, won by his Australian driver Daniel Ricciardo, that a decision would have to be made by "end of June, beginning of July".
"We're waiting with great interest to see the relative performance of engines in Montreal in two weeks' time," he added.
Canada, on June 10, is the seventh round of the 21-race season and both manufacturers are planning to bring major upgrades to the Circuit Gilles Villeneuve.
Red Bull, whose Renault engines are branded Tag Heuer, have won twice this season but have had a rocky relationship with the French company since the start of the V6 turbo hybrid era in 2014.
Honda are working with Red Bull-owned Toro Rosso and have improved the performance of their engine notably since the termination of a three-year partnership with McLaren.
While Ricciardo is third overall in the championship with 72 points, he has suffered reliability problems - as well as a costly crash with team-mate Max Verstappen - and is 38 points off Mercedes' leader Lewis Hamilton.
Ricciardo started on pole position in Monaco, the team's first in two years, and won despite having a power unit problem from Lap 28. Verstappen set the fastest race lap, the fourth time in six grands prix that Red Bull have done that.
Ricciardo is out of contract at the end of the season and is also waiting to see what engine Red Bull go for.
"First thing is get the engine sorted and then very much follow on from there with driver," said Horner.
Reigning world champion Hamilton predicts that Ricciardo will ultimately remain at Red Bull next season, and that Finn Kimi Raikkonen, 38, will stick to Ferrari.
"I would imagine his future is there," said the Briton of the Australian. "He is one of the top drivers so I am sure there will be options for him. There's only Ferrari and Mercedes that he could ever consider, but currently here that's unlikely. I don't think that's going to happen.
"I imagine Kimi's going to want to continue (at Ferrari). Why not? He's driving really well this year and he doesn't seem to be ageing so I'm sure he'll go for a little bit longer."