“Their straight-line speed has taken a significant step recently, and I think that whereas we could match them with smaller wings previously, now we can’t get near,” he said.
“We saw that particularly at this circuit where Lewis, in particular, had a significant straight-line advantage with a bigger rear wing on 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.”
Horner also said his team would be raising the situation with the FIA, while adviser Helmut Marko added: “Something has been weird since Silverstone. So it is now up to us to research why they are suddenly so fast. Is it just the engine or something else?
“We now have to work day and night to uncover its secret and then counter it. As quickly as possible. It is up to us.”