Christian Horner says Red Bull may not go ahead with Renault's mooted engine upgrade for the Russian Grand Prix, saying it needs to determine whether it is 'worth taking a penalty for'.

With relations between the two parties now almost irreparably strained as the latter stages of the season approach, Red Bull had been expected to make use of an updated Renault power unit for the Russian Grand Prix in Sochi.

However, as speculation grows that Red Bull will not use Renault engines at all in 2016, Horner questions whether the performance gains from the update will be worth taking another penalty for with only a few races remaining

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"It hasn't been confirmed to us when it will be or how much performance it will be either," he said. "When we understand what the performance gain is then you've got to weigh it up with, 'Is it worth taking a penalty for?' So we'll wait to hear from Renault on that."

With Renault suffering more reliability woes in Monza after Daniel Ricciardo's brand new power unit broke after just five laps, Horner admits he is impressed by rivals Mercedes and its upgraded power unit.

"[The upgraded Mercedes engine] looked quite frightening on Friday and then obviously they must have had some kind of issue with Rosberg, because it looked like they wound the engines back a little bit. But certainly the step they made on Friday look significant, quite worrying. I mean, they're in a league of their own at the moment."