Horner believes some of the explanation for Max Verstappen being unable to challenge Valtteri Bottas for victory in Sunday’s race can be put down to Mercedes making a step forward with its straight-line speed performance. 

And the Red Bull chief suspects Mercedes’ latest specification of engine that was introduced at the Belgian Grand Prix is behind the power boost. 

"Their straight-line speed has taken a significant step recently,” Horner said. “Whereas we could match them with smaller wings previously, now we can't get near.

"We saw that particularly at this circuit, where Lewis in particular had a significant straightline advantage with a bigger rear wing of the car.

"We've got to maximise our package as best we can. And, as I say, it's surprising that they've made the step that they have with the power unit.”

Mercedes’ advantage over the Red Bull cars was worth as much as 20kph at points across the weekend at Istanbul Park, Horner claimed.

“You can see across all of their teams, they’ve obviously got some reliability issues that they’re managing, which is unusual for Mercedes,” he explained. 

“But the performance is still incredibly impressive. Whether one comes at the expense of the other, I don’t know.

“But certainly today it’s been highlighted I think if you look at some of the speed deltas on the back straight, at some points it was 15 to 20 kilometres an hour which is more than if there had been DRS.”

However, Toto Wolff dismissed the suggestion a trend is beginning to emerge that suggests Mercedes currently has the fastest car. 

"I don’t think you can say that with absolute conviction,” Wolff said. “I believe that we have a really good package now. The car is competitive and was very competitive this weekend. 

“If it wasn’t for a grid penalty I think we could have had every chance to finish first and second and that would have meant eight points more. 

“I’m looking carefully optimistic at the future, Austin is a track that we like and it’s another opportunity, but I’m under no illusions that this is going to go very much to the end in the driver’s championship.”