Red Bull boss Christian Horner says the progress of Renault and Honda's respective engine upgrades in the lead up to June's Canadian Grand Prix will dictate the team's decision on 2019.
Red Bull is in the final year of its contract with Renault and it looks likely the pair will split, having seen its relationship deteriorate since the introduction of V6 turbo engines in 2014. The only alternative is Honda, who supplies junior team Toro Rosso. Red Bull started formal discussions with the Japanese manufacturer after the Azerbaijan Grand Prix.
Renault previously warned Red Bull it would not budge on its deadline of May but Horner has other ideas.
"There won't be a decision in May," Horner said. "I think potentially both engine suppliers are bringing an upgrade around Montreal so we wait and see with interest."
Renault has not officially confirmed when it will bring an upgraded spec, but Horner believes it will be there for the seventh round of the season on June 10.
"Hopefully around Montreal time, there is a next [Renault] engine introduction; we are hoping for a performance increase."
Honda has welcomed Red Bull's decision to delay the deision. The Japanese manufacturer is yet to decide whether it is willing to take on the additional workload of supplying two teams in one season.
"We regard it as a positive thing, giving us time means we have time to consult more," Honda boss Masahi Yamamoto told Autosport. "We are very positive.
"To deal with Red Bull is a really big thing for Honda, because they are a top team. This gives us a sort of pressure as an engine manufacturer. We have to consider many aspects very well, can we really deal with Red Bull's size? We have to feel the responsibility to supply the engine."