Formula 1 motorsport boss Ross Brawn says he does not wish to “speculate” about the latest FIA technical directive following scrutiny over Ferrari’s power unit.

Significant gains with its engine and straight-line speed has helped Ferrari turn in a strong resurgence since the summer break by claiming six straight pole positions and winning three races in Belgium, Italy and Singapore.

Its apparent surge in engine performance has raised suspicion among its rivals, with Red Bull approaching the FIA to request clarification over a proposed fuel system solution that would have been deemed illegal. The FIA subsequently issued a technical directive to clear up the matter.

Comments made by Red Bull driver Max Verstappen after the United States Grand Prix suggested Ferrari had struggled in Austin due to being forced to make changes to its power unit, an allegation which irked the Scuderia.

Speaking about Ferrari’s struggles, Brawn commented: “The United States Grand Prix turned the clock back by six months, as not since Barcelona had we seen a podium without a Ferrari driver on it.

“In Spain, Mercedes scored a one-two finish that appeared to definitively stamp their authority on the battle for both championship titles.

“But then, especially after the summer break, Ferrari seemed to make a breakthrough as they scored six pole positions and three wins but in the end the Maranello team went backwards, and in a surprising way.

“Not so much in qualifying where Vettel was just 0.012s behind Bottas, but definitely in the race, where Leclerc finished a whopping 52 seconds behind the Finn.

“The Ferrari man struggled particularly in the first stint on Medium compound Pirellis, lapping around a second down on the leader. Once he moved to the Hard tyre, Leclerc was more competitive, but by then his chances of competing for a podium position were long gone.

“From the outside, it’s hard to explain the drop-off and I certainly don’t want to speculate about the latest FIA technical directive relating to fuel flow metering,” he added.

“What is clear, though, is that Ferrari struggled in Texas, especially when it came to tyre management. It wasn’t so much about making them last, but more about getting them to work properly.

"It’s true track conditions changed a lot over the course of the weekend, as the temperature went up significantly, but it’s equally true that Mercedes and Red Bull did a better job of adapting to the changing conditions.

“There’s a lot to do in Maranello over the coming days to analyse, reflect and come up with countermeasures, especially as this will also be useful for next year.”