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Spain - Renault wants Red Bull to decide by May which engine it will use in 2019.
Red Bull is making no secret that it could follow its junior team Toro Rosso in switching to Honda power at the end of the season.
Boss Christian Horner has already expressed some concerns that Renault is prioritising reliability over performance for 2018, at least early in the season.
Renault's Cyril Abiteboul said that is only understandable, given the changes to the long-life engine rules this year.
He said in Barcelona: "In such a situation, the ordering of priorities is obvious.
"It is important to understand that that reliability gives more options for increasing efficiency. When you increase reliability, you can use more and more power.
"The situation (in 2018) will be comparable to the end of last season in Abu Dhabi, when we were able to use the maximum potential of the engine."
Red Bull will therefore assess which option - Renault or Honda - will give them more power for 2019 and beyond.
In need of an answer
Abiteboul warned: "We cannot wait forever.
"I know that Christian has said he has options, and he is absolutely right. Like him, I know and read contracts.
"But we have obligations to help formula one, not Red Bull, and it is obvious that we must plan our work.
"The deadline is spelled out in the sporting regulations, because as far as I know, there should be clarity as to which engine supplier each team will have for the following year by the end of May.
"As far as I can tell, this is our deadline. By this time, we want to know whether the cooperation will continue," Abiteboul added.