Red Bull fears it cannot catch F1 rival Mercedes in 2015

As efforts to relax F1's engine freeze appear to have failed, with Mercedes refusing to concede ground, Red Bull's engine partner Renault now has a narrower window in which to improve their power unit

And although Red Bull and Renault have restructured their partnership to try to ramp up their development, team chief Christian Horner says he is not expecting to be able to overhaul Mercedes next season.

"We are not writing off next year but, for sure, there is a gap there," he said. "And, from what we hear, the performance gains that Mercedes are bringing next year, it is going to be a massive challenge to reduce that

"Renault can make an inroad but I think it would be unlikely to match this year's [Mercedes] engine.

"I think we can get closer to it, but Mercedes are not going to stand still and Ferrari are in a similar situation. It is very tough when the engines are frozen at the end of February."

There are also mounting concerns that Mercedes could even step further ahead of Renault and Ferrari next season, with improvements expected to come from its energy recovery systems and exhaust areas of the car.

Horner says that he has heard suggestions Mercedes' 2015 power unit will be much better than this year's, which would leave Renault and Ferrari still playing catch up.

"I think if you believe the numbers that are floating around, the gap could actually grow rather than diminish," he said, in reference to speculation Mercedes could find around 70bhp more.

"That would be rather depressing if that was the case."