Comparisons of the respective Renault and Honda engine upgrades at the Canadian GP will decide who Red Bull choose for 2019, team boss Christian Horner claims.

The Anglo-Austrian team has been in talks with both parties but their current supplier, Renault have expressed frustration by slamming what they consider a lack of loyalty and commitment from their long-time partner while also wanting a quick decision.

Well aware of the importance of getting it right, Red Bull are keen to take their time but the next race in Montreal is an important one in the annual engine development cycle.

"It will be a month or so. End of June, beginning of July. It's the time-frame we've always talked about," said the Briton.

"We want to compare the two engines in Canada. We'll get small updates in Montreal but we need to get the most out of them. It's very important to us."

Problems with reliability and the inability to match Mercedes and Ferrari in qualifying are the main reasons why Red Bull is considering a swap to an engine that was ditched by McLaren after 2017 and hasn't been hugely competitive this year in the Toro Rosso.

But Honda does have greater resources than the French manufacturer and Brendon Hartley is expecting a strong improvement in Canada.

“I’m still pretty positive for Montreal,” he said. “We have an engine update coming, and we had a new floor here so we had some performance to put on the car.

“It looks like the team is really starting to unlock the car’s potential.”

On the engine specifically, the Kiwi added: “I don’t know exact numbers, I think it’s better you ask someone from Honda or Toro Rosso, but I think it’s meant to be a sizeable step.”