Red Bull Racing team principal Christian Horner has questioned Mercedes' sudden boost in straight line speed after the Silver Arrows introduced new power unit parts at recent races in Sochi and Istanbul. The reigning champions have a clear advantage on the straights and Horner has put it forward to the FIA to have a look.

The Austrian team were the clear front runners in the first half of the campaign in terms of pace, but Mercedes seem to have closed the gap and moved ahead at recent races. Neither team is allowed to introduce upgrades and Mercedes have made it clear that all they have done is introduce a new engine, without any upgrades.

"It's surprising they appear to have made the step they have with the power-unit," Horner said, as quoted on Sky Sports. "The straight-line speed of Lewis was 15 to 20kmh up after the kink on the straight, which is phenomenal."

"When you've got straight-line speed that's greater than with a DRS open, that's pretty impressive," the Red Bull chief added. "It's down to the FIA to continue to look at these things."

Max Verstappen, who took the lead in the championship after the Turkish Grand Prix on Sunday, has called on his team to "step up" in light of Mercedes' recent resurgence. The most recent races at Monza, Sochi and Istanbul were expected to suit the Brackley based team, but their straight line speed has put Red Bull on alert going into the final six races of the campaign.

"Of course overall I think we have been at tracks that naturally were a bit better for Mercedes but then this track was a bit unknown and clearly they were ahead of us," Verstappen told Sky F1 at Istanbul Park. "I do think we need to step it up a bit to be in the fight until the end of the season."

The Dutch racer finished a distant second to Valtteri Bottas in Turkey, while also being fortunate to finish in second in Russia after starting at the back of the grid. The F1 circus will next travel to the United States for the race in Austin, a track at which both Mercedes and Red Bull seem to run well.

Max Verstappen
Red Bull's Max Verstappen was a winner in Austria last year William WEST/AFP