Red Bull reckons its Formula 1 engine supplier Renault made a mistake by not focusing development of its 2014 power unit around one car.
The world champion squad endured its worst race of the season in Austria last time out, leading team boss Christian Horner to brand Renault's performance "unacceptable".
Speaking to reporters ahead of this weekend's British Grand Prix, Horner suggested Mercedes and Ferrari had gained an advantage on Renault by building their engines specifically for their works teams.
When asked whether Renault had focused on too many of its customers, rather than works partner Red Bull, Horner said: "That can be an element of it because you are never going to be able to satisfy everybody.
"When you look at the engine Ferrari has made, the customers have had to adapt their cars accordingly.
"Mercedes likewise, whereas Renault has tried to keep all of their customers happy, which is an admirable thing to do, but it's not the best way to be competitive."
CONCERN DATES BACK TO 2012
Horner revealed that Red Bull became worried about Renault's readiness for this year's engine regulation changes as far back as the end of 2012, when Sebastian Vettel claimed his third straight world title with Renault V8 power.
"We raised our concerns as far back as the end of 2012 with the direction the project was going," added Horner.
"But designing and building engines is not our core competence - nor should it be - we're a chassis manufacturer.
"And I don't think any of us could have envisaged that Renault would be as far behind as they have been."
Renault's focus may naturally shift in Red Bull's favour for next season, amid growing uncertainty about whether Renault customer teams Lotus and Caterham will switch supplier, and Horner said Red Bull remained committed to helping Renault improve.
"We've had some discussion with Renault and they fully accept the situation they're in is not where we should be," Horner added.
"All we can do is try to help and support them with the resources we have to try and improve things.
"Obviously, the difficulty we have within the regulations as they currently are is that it's very difficult to make significant changes.
"[But] I think fuel is going to be a key area of development - Total has got some stuff in the pipeline that looks encouraging, but Renault is only just starting to catch up to where Mercedes were six months ago.
"We just need to make sure we engineer ourselves out of the situation we're currently in, and that's fundamentally supporting Renault to get more out of the power unit."
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