Jenson Button says it is only a matter of time before Lewis Hamilton shows his true speed for his Mercedes team and that it will "shock you all".
Hamilton has struggled for pace more than expected compared to team-mate Nico Rosberg since joining Mercedes this year.
But his former McLaren team-mate Button said: "For Lewis, it is the same as it is for all of us - if he does not have a car balance that suits him, then it is not easy.
"And, of course, Nico is fast. But Lewis has not lost his speed. It is still there. He will come back and then he will shock you all."
Button was talking in an interview with the German magazine Auto Motor Und Sport conducted at the Canadian Grand Prix, the day before Hamilton outqualified Rosberg for the first time in four races.
Button added: "You have to be fair. He has at least scored significantly more points than Nico. Lewis is incredibly fast."
Button was asked whether he thought Mercedes had been helped by the controversial 1,000km tyre test the team did with Pirelli before the Monaco Grand Prix, and for which Mercedes and Pirelli face a hearing of the FIA international tribunal on Thursday.
He said: "Both drivers say no. You have to believe them. Otherwise they would be liars. So I believe them."
Button's McLaren team have had a disappointing year, with a best result of fifth place, but he said it was important to keep trying to develop their current car rather than give up on 2013 and focus on 2014, when F1 will introduce a new engine formula and some revisions to the chassis rules.
"The cars in 2014 are slightly different, but we must first understand the current car to make a step forward in the car for next year," he said. "And also we want to win races this year."
He said he was looking forward to next year's introduction of 1.6-litre V6 turbos with extensive energy recovery, which will replace the current 2.4-litre naturally aspirated V8s.
"2014 will be completely different for all of us," he said. "Already this year, every driver has to learn a whole lot about how a Formula 1 car works and how to take care of the tyres. This is bad for me because I've always worked on such things.
"Next year is again a lot of new things: The new powertrain, the new aerodynamics, such as the Kers (energy recovery) and how the turbo works.
"We get power from three different sources (the engine, the energy recovery and the turbo). The speeds will be unfamiliar.
"It is probably the biggest change we've had in Formula 1. So we have to cope."