Rival top teams are dubious about the performance of Ferrari's 2019 engine.

At Spa and Monza, where Charles Leclerc secured his first two victories back-to-back, the superiority of the Maranello-made power unit was clearer than ever.

"Right now, Renault, Honda and ourselves are at about the same level," Mercedes' Toto Wolff told Auto Motor und Sport.

"Only Ferrari stands out, and in a pretty serious way."

There are various theories floating around in the paddock. One suggests that Ferrari is generating more than the allowed kilowattage from the battery.

Another is that the engine is managing to store and then use extra fuel at certain moments between the mandatory sensors, while a third rumour is that the engine is burning extra oil for a power benefit.

An unnamed rival team member said: "They must have come up with something very clever.

"If it turns out to be legal, all we could do is take off our hats."

Red Bull team boss Christian Horner said: "We sent a number of questions to the FIA, but have not received any answers."