Red Bull Team Principal Christian Horner has suggested Honda and current supplier Renault may never close the engine performance gap to Mercedes and Ferrari during the current engine formula.

The pecking order between the four suppliers has not changed since the introduction of the current V6 hybrids in 2014 and that has been the main reason for Mercedes' continued success and Red Bull's ongoing struggles.

Although the gap between them has shrunk, as the technology is still immature the room for development remains large for all and that means the likelihood of the units finally settling at around the same level in the short-to-medium term is small.

Indeed, for the first time, Horner has acknowledged his and other teams not using the class-leading engines may have to wait for the new rules in 2021 before they can consider competing at the very 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 said talking to 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."

So influential are the engines in determining performance, McLaren is considering ending their re-collaboration with Honda after just three years as the Japanese manufacturer can't reach a level that allows the British team to fight where their chassis deserves. 

Though it is thought Red Bull junior team Toro Rosso will become their sole customer should they decide to divorce, it is this complexity and inability to be competitive that has kept several big names away.  

For the engines being considered for 2021, however, that wider interest is back with several names mentioned but there remains a debate as to just how road relevant and efficient the new formula should be.  

"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, referring to another key issue that is trying to be addressed.

"Aston Martin is 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."