Red Bull team principal Christian Horner has questioned how Mercedes have seemingly made gains with their speeds during the season.
Under F1’s rules, teams are not allowed to improve the power of their engines mid-season, but Horner insists Mercedes’ straight-line speed has "has taken a significant step recently".
Mercedes have rejected the claims, saying nothing has been changed other than fitting a new engine to Lewis Hamilton’s car for last weekend’s Turkish Grand Prix.
Both Hamilton and Red Bull’s Max Verstappen have taken new engines over the last couple of races, resulting in grid penalties for the pair.
Verstappen currently leads Hamilton by six points in the race for the Drivers’ World Championship, with six races to go after the Dutchman finished second in Sunday’s Grand Prix, with Hamilton securing fifth.
"It's surprising they appear to have made the step they have with the power-unit," Horner said.
"We could match them with smaller [rear] wings previously; now we can't get near. And we saw that particularly at this circuit, where Lewis had a straight-line advantage with a bigger rear wing on the car.
"You can see across all their teams Mercedes obviously have some reliability issues they are managing, which is unusual for Mercedes. But their performance is still incredibly impressive.
"Whether one comes at the expense of the other, I don't know, but if you look at some of the speed deltas on the back straight at some points it was 15-20km/h, which is more than if there had been DRS (overtaking aid in use)."
Mercedes have refuted Horner’s suggestions, insisting they are not backed up by the data.
Red Bull’s claims come after Hamilton himself made suggestions about his rival’s Honda engine earlier in the season, after they looked to have made a step forward in their performance around the time of the French Grand Prix.
Mercedes boss Toto Wolff admitted the company were having issues with their engines, and were having to juggle the issue of reliability against performance.
"When you start to push the boundaries with power-unit performance, at a certain stage you will experience some obstacles," he said.
"Our power-units have been the most reliable since the introduction of the hybrid engines in 2014 and because the boundaries are being pushed we have seen examples of, let's say, unusual noises within the combustion engine that aren't completely understood at that stage yet.
"They caused some troubles in the past; we had some engines that basically failed and now it's about containing the problem because changing parts is not something you would tackle."