Outdone by Mercedes at the start of this season, Christian Horner fears Red Bull are paying for Renault's late start to the new V6 era.
This season has been all about Mercedes and Mercedes power while Red Bull's Renault engine have struggled with reliability and power.
The Renault-powered teams all had a torrid pre-season, spending a great deal of it on the sidelines due to engine and cooling issues.
Once on track in Australia, the situation did improve and has steadily since, however, Renault are still lagging some way behind Mercedes.
And Horner believes that is due to the French manufacturer making a late start in the development of their 1.6-litre V6 engine.
"It's quite simple really: we've had a massive engine regulation change and Renault have turned up and they weren't as ready as some of their competitors were," the Red Bull Team Principal told Sky Sports F1.
"So we've been playing catch-up. They're working tremendously hard at it and we're slowly closing that gap down.
"As you can see we've got a very good chassis, I just think that they started too late. It's as simple as that."
Pressed as to why he believed Renault had made a late start, the Red Bull team boss said: "That's a very good question and perhaps one you should ask Renault because they were the guys who pushed very hard for this regulation change, so one would have thought that they'd have been the most prepared for it.
"The guys are having to recover lost ground and they've got a recovery plan, they're working to that, they're trying to close that gap.
"Mercedes, take nothing away from them, they've done a fantastic job and turned up with a very strong engine unit this year."
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