Red Bull team principal Christian Horner is concerned Mercedes and Ferrari will remain dominant in Formula One until the next set of regulation changes in 2021.

Under revised engine rules brought in for this year, Mercedes - constructors' champions in the previous three seasons - and Ferrari have proven a class apart.

All but one race in 2017 has been won by those two teams, with victory for Red Bull's Daniel Ricciardo in Azerbaijan proving the exception.

Horner expects his Renault-powered outfit, who enjoyed their own period of superiority prior to Mercedes hitting the front, will continue to have the odds stacked against them for the next three campaigns.

"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," Horner told Sky Sports F1.

"But the reality is, those two manufacturers 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."