Red Bull's Christian Horner fears Renault and Honda will not be able to catch up to Mercedes and Ferrari before F1's engine rules reset in 2021.

The Mercedes and Ferrari works teams are dominating this year's championship, with Renault-powered Red Bull third in the standings having scored fewer than half the points of the Constructors' Championship leaders.

The Renault and Honda engines have both been beset by reliability problems this year and Horner reckons both manufacturers will continue to struggle to bridge the power gap to the front.

"We'll never accept that we can't be competitive so we'll keep pushing and keep developing and try and make up whatever horsepower deficit there is on the chassis side," the Red Bull team boss told Sky Sports F1.

"But the reality is, those two manufacturers [Ferrari and Mercedes] have such a march, such committed investment, it's difficult to see how the others will catch up in the intervening period between now and 2021."

Horner is a long-time critic of the hybrid turbo engines introduced in 2014 and said at the Italian GP that "this engine has done nothing positive for Formula 1" after the Monza grid was scrambled by power unit penalties.

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F1 has committed to keeping the current engines until 2020 and talks have been ongoing since the start of the season about what the next-generation of rules should look like.

With moves to make the next engine formula cheaper and less complex, Porsche have confirmed their interest in returning to F1 as a supplier, while Horner says the likes of Aston Martin, who Red Bull work with on a hypercar project, and Lamborghini could also be tempted.

"We may well [lose manufacturers] but I think there are iconic manufacturers who would be keen to come in if it was affordable," said Horner.

"Aston Martin being one of them, Lamborghini attending meetings. So long as you have Ferrari there, so long as you have historic teams like McLaren and Williams and so on, and other manufacturers like those I've mentioned were to come in. I think it's all about the spectacle.

"Manufacturers have always come and gone in F1 whenever it's suited them. I think the most important thing is to get the product right, get the show right, and then it's up to the manufacturers to be there or not."