Honda will deliver a winning engine despite the slow start to its Formula 1 return, Mercedes' engine boss says.
Honda's engine has been F1's least competitive at the start of the company's new relationship with McLaren this year.
The team are targeting race wins in 2016 and the title with Fernando Alonso, who has a deal until 2017.
Mercedes' Andy Cowell told BBC Sport: "They are McLaren-Honda. They can win world championships."
McLaren have so far failed to score a point in 2015, and Alonso and team-mate Jenson Button managed to progress beyond the first knockout part of qualifying for the first time only at the last race in Spain on 10 May.
But Cowell added that he was impressed by Honda's strategy so far in F1.
"The way they are going about it, Honda are formidable - in their approach, investment and determination," he said.
And Cowell said Honda would not be affected by a quirk of the engine rules that has restricted the amount of engine development it can do this year compared to its rivals.
Mercedes, Ferrari and Renault, whose engines all raced last year, have been allowed to use 32 tokens - out of the 66 that define the entire engine - over the course of the winter and the 2015 season.
But Honda, as a new manufacturer, has been allowed to use during the championship only the average of the tokens that were unused by the other manufacturers at the start of the season, which comes to nine.
Mercedes had seven remaining at the start of the season, after using 25 over the winter; Ferrari had 10 for the championship, after using 22; and Renault 12 for the season after using 20.
Next season, Honda will be on a par with the other manufacturers in being allowed to apply 25 development tokens to the engine which finishes the 2015 season.
Asked how much this compromised Honda, Cowell - whose Mercedes engine is the best in F1 - said: "I don't think they are. It's 25 tokens for next year. We changed a huge amount of the power unit and used 25 tokens.
"Thirty-two was super-generous. Twenty-five is enough to change everything, to be honest.
"And I would say they have got a reliability challenge which doesn't require any tokens [to fix].
"I think they're doing the right thing. They're playing medium to long-term. They're not playing this season. This is R&D in the public domain."
The Honda engine is estimated to be in the region of 110bhp down on the Mercedes, having started the season adrift by double that amount.
Honda has managed to achieve that progress without using any of its development tokens so far.
A spokeswoman said the company was "satisfied" with the number of tokens it had been permitted.
As well as improving reliability, which allows the engine to be used more aggressively and therefore ensures that more power is generated, Honda also has to improve driveability.
McLaren racing director Eric Boullier said that when developing an engine and trying to catch those ahead "the last horsepower or kilowatt is more difficult. And in the middle of this you have this drivability grey area which is very complicated, and which is costing a lot of performance."