Christian Horner says Red Bull Racing remains open about its engine choice for 2019, with a decision expected in the second quarter of next year.
The Milton Keynes-based outfit will still be powered by Renault next season but the addition of McLaren as the manufacturer's customer, in replacement of Toro Rosso which itself is switching to Honda, is likely to spell the end of the Red Bull-Renault.
At this stage, Red Bull is keeping its options open for 2019.
"Well, everything is open for 2019. Nothing is fixed," Horner said in an interview with Formula1.com.
"We will make a decision around spring to early summer next year."
Horner will obviously devote a lot of attention to Toro Rosso-Honda's development and performance next year, with the Faenza squad perceived as a possible test bed for Red Bull as it considers a potential Honda partnership in the future.
"Renault has always provided a fair service. Every world championship, every Grand Prix victory we have achieved has been with a Renault engine – as a customer," Horner added.
"They have always been very fair in providing equal services, so we have faith that this will continue.
"Yes, it is great that Honda remains in the sport and for sure, it is a great deal for Toro Rosso being a works team.
"And, of course, from a Red Bull Group perspective it gives the opportunity to follow the progress of Honda before having to make a decision in springtime next year what our engine is for the future. "
In the interim, given the prospect of a parting of ways at the end of 2018, Horner is confident Renault will deliver a competitive and reliable product to Red Bull next season despite the rivalry with McLaren and Renault's own works team.
"McLaren will go through a natural honeymoon with Renault. Fernando Alonso was their driver and so on, but we need to focus on our own business.
"As long as we get a fair service from Renault, that’s all we ask.
"And, of course, they want to bring up their own team, so we rely on their integrity as in all the previous seasons – in good and bad times, they have always provided parity of equipment."