London - The Canadian Grand Prix could determine whether Red Bull decides to stick with Renault, which powered the team to all of its 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, has won twice this season but the team has had a rocky relationship with the French company since the start of the V6 turbo hybrid era in 2014.
Honda is working with Red Bull-owned Toro Rosso and has improved the performance of its engine notably since the termination of a three year partnership with McLaren.
While Ricciardo is third overall in the championship, 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 also waiting on decision
The Australian is out of contract at the end of the season and is also waiting to see what engine Red Bull goes for.
"First thing is get the engine sorted and then very much follow on from there with driver," said Horner.
"I think we've had a great chassis from the first race, to be honest," he added. "Our problem has always been on Saturdays... if we can improve our Saturdays, then our races will be competitive.
"If we could just get a little bit more at the end of Q3 (the final phase of qualifying) on peak power, there's no reason why we shouldn't be able to give Ferrari and Mercedes regularly a hard time."
Ricciardo started on pole position in Monaco on Sunday, 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.
"The FIA understand the situation we're in and there's no hard and fast deadline," said Horner of the engine decision.
Renault has previously said it would need to know by the end of May because of the long lead times involved in manufacturing.