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London, England - Formula 1 world champions Mercedes remain in a league of its own after showing a "worrying" step up with their latest power unit developments, according to Red Bull principal Christian Horner.
Double world champion Lewis Hamilton won the 2015 Italian Grand Prix with a 25-second margin over Ferrari runner-up Sebastian Vettel.
'League of their own'
In the Italian GP first practice, the Briton lapped more than a second and a half faster than Vettel - whose car was the quickest after the Mercedes pair of Hamilton and Nico Rosberg.
Horner said: "It (the Mercedes engine) looked quite frightening during first practice and then obviously they must have had some sort of issue with Rosberg because it looked like they wound the engines back a little bit"
"But certainly the step that they made on Friday (September 4) looked significant, quite worrying. I mean they are in a league of their own at the moment."
Rosberg had to revert to an engine that had been used for five grands prix prior to Sunday's race due to a cooling problem. The unit then blew two laps from the end and he finished third.
New engine suppliers
Horner, whose team's relationship with partners Renault now looks doomed despite one year remaining on the contract, said the race gave a glimpse of Mercedes' real speed when Hamilton was told to push hard at the end.
Horner said: "We saw the kind of pace that Lewis had at the end there and it was pretty awesome, so I guess he was just managing the gap throughout the Grand Prix...they're clearly some way up the road."
Red Bull has been linked to both Mercedes and Ferrari as they sound out other potential suppliers for 2016, with the latter having said as far back as June they would be willing to step in.
Mercedes has been considerably less keen, with Hamilton making clear his opposition, and reports after Monza indicated the German automaker's board had ruled out supplying Red Bull.
Horner said he was still waiting to hear from Renault about their plans.
Renault is assessing their future in the sport, and whether to become more involved by buying into a team like struggling Lotus or quitting altogether.
Horner said: "Obviously, time is starting to press on now, we're now into September and everybody needs to know what are Renault plans for the future. So I would have thought within the next two weeks we should all know what Renault's position is."