Red Bull is fully expecting Mercedes to prove a thorn in its F1 title-challenging side at the upcoming three grands prix, starting with next weekend's race at Silverstone.
Lewis Hamilton has won the last three British Grands Prix, with last season's event a controversial affair following his incident with Max Verstappen that resulted in a spectacular crash for the Dutch driver.
Hamilton heads into his home race 98 points adrift of Verstappen in the current standings and seemingly with little chance of making it four in a row at Silverstone given the performance of the Mercedes W13 so far.
Yet Horner is convinced Mercedes poses a significant threat, not just next Sunday but also at the subsequent races in Austria and France given the smoother nature of all three tracks that will aid Mercedes' cause.
Asked whether Mercedes could be a factor in the upcoming British-Austrian double-header, Horner, replied: "Yeah, definitely. I think they will be strong at Silverstone, I don't see any reason why they wouldn't be. Paul Ricard the same.
"Ferrari will be quick at Silverstone as well. It could be more of a challenging circuit for us due to the nature of its layout.
"It is incredible that we have put a run of six race victories together. Silverstone could well be the most challenging."
Horner expecting Red Bull tail off
Verstappen has won five of those six races to open up a 46-point cushion over team-mate Sergio Perez in the drivers' standings, with Ferrari's Charles Leclerc a further three points adrift.
Declaring reigning champion Verstappen to have "hit a purple patch of form", Horner is fully expecting his team's remarkable run to come to an end sooner rather than later.
"I would love to continue but with every race, it gets harder," said Horner.
"The swing in the championship from where we left Australia [Verstappen 46 points behind Leclerc] to where we are, it just shows how quickly things can move."