Honda's shock announcement on Friday that it will depart at the end of 2021 leaves the pinnacle of motorsport as a series with only three engine manufacturers and no sign of any newcomers.

Car companies have come and gone throughout the 70 year history of the sport, Honda most recently selling its team and pulling out as a constructor in 2008 after a global financial crash.

This time, however, the justification -- to focus on zero-emission technology such as fuel cells and batteries -- feels more significant.

It is a question of perception as well as cost, and that can be hard to change.

"Formula One is now at risk of becoming slightly irrelevant to car manufacturers, and particularly car manufacturers who are not yet vested in the sport, because the world of vehicles is changing much faster than even five years ago people thought would be the case," former Cosworth F1 engine head Mark Gallagher told Reuters.

"I think the Honda move, whilst being shocking, is entirely in keeping with what we are seeing happening around the world," added the former Jordan, Jaguar and Red Bull marketing executive.

"For Formula One it poses some pretty fundamental questions about what they are going to do over the next three or four years."

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All-electric Formula E, by contrast, counts 10 manufacturers this season -- including Germany's big four of Audi, BMW, Mercedes and Porsche.

The costs are far less in Formula E and they also have a 25-year exclusive deal as the governing FIA's sole recognised electric single-seater series.

Series founder Alejandro Agag told Reuters that while combustion motorsport had a future, electrification was an unstoppable trend.

"Things like the COVID pandemic have actually accelerated that process because the whole business community is talking about the fact that the world post-COVID should be a greener place," said Gallagher.

"Formula One finds itself in a slightly unenviable position of being wedded to fossil fuel-based technologies at a time when the rest of the world is moving in a different direction."

The sport's next change of engine regulations is set for 2026.

It set out a sustainability plan last year, aiming for a net zero-carbon footprint by 2030, but still needs to do more to trumpet its achievements.

The current V6 turbo hybrid power engines, in use since 2014, boast thermal efficiency of more than 50% and produce more power from less fuel than any others.

Renault F1 boss Cyril Abiteboul said the question of relevance was key, and the sport needed a clearer message "to win the battle of perception".

He told Reuters Formula One needed to find its own style and space.

"Technology is only good enough if you also manage to get your message across and that's what we are failing to do," said the Frenchman.

"I have a concern that when you lose a battle of perception its very difficult to turn this around. We may have to accept a sufficient departure from the current engine/powertrain such that we can also reset the perception of what we do."

Abiteboul felt that meant accelerating on sustainability and economics.

"Stefano Domenicali has that topic to address with the utmost priority, that's for sure," he said of the former Ferrari team boss and Lamborghini CEO, who takes over from American Chase Carey.

By Alan Baldwin