Red Bull boss Christian Horner has labelled the progress that Mercedes has made with its new engine as 'frightening'.
Lewis Hamilton and Nico Rosberg were handed an upgraded power unit for the Italian Grand Prix weekend, with their Mercedes bosses having elected to use all seven of their in-season development tokens.
The potential of their improvements, which are aimed at new development paths the team wants to take for 2016, was clear from first practice when both Mercedes drivers were in a class of their own.
And although Rosberg's weekend was derailed when his engine hit problems and he had to revert to a previous specification unit, Horner is under no impressions about the potential Mercedes has shown.
"It looked quite frightening on Friday and then they must have had some kind of issue with Rosberg because it looked like they wound the engines back a bit," said Horner.
"But the step on Friday looked quite significant, quite worrying. They are in a league of their own at the moment."
Horner's views on the Mercedes step must be particularly painful, because his outfit had been hoping to secure a supply of the company's power units for 2016.
However, as Motorsport.com revealed on Monday, Daimler chairman Dieter Zetsche met with Toto Wolff and Niki Lauda over the Italian Grand Prix and has decided not to accept any overtures from its rival for a customer deal.
Although Ferrari's own updated engine appeared to have made progress in qualifying, as Kimi Raikkonen and Sebastian Vettel took second and third on the grid, in the race Hamilton again showed the edge.
Horner added: "Certainly in qualifying they were closer than Friday suggested but then in the race we saw the pace Lewis had on a scrubbed prime tyre at the end there. It was pretty awesome.
"I guess he was just managing the gap throughout the grand prix. They [Mercedes] are clearly some way up the road."