Formula 1's switch to 100% sustainable fuel from next season is proving much more expensive than expected, according to Mercedes team chief Toto Wolff.
The cost of the fuel was raised by one of the engine manufacturers at a recent F1 commission meeting that includes team bosses and major stakeholders.
"What makes it so expensive is that the whole supply chain and energy contribution needs to be green," Wolff told reporters at the Miami Grand Prix.
"To achieve all of that, you need a certain specification of ingredients that is very expensive. And it's coming in much more expensive than anyone thought.
"So we need to look at whether there's anything we can tweak to bring the per-liter price down."
Wolff said Mercedes' fuel partner Petronas was fully committed technically but looking into whether a regulation change could make sustainable fuel also more financially sustainable.
Red Bull boss Christian Horner agreed there were a lot of development costs involved, but said it was not a significant issue for his team.
"Maybe a certain bracket should be introduced going forward. But fuel is potentially one of the bigger performance differentiators. The fuel companies seem very engaged in that," he added.
F1 plans to be net zero carbon by 2030, a plan covering the cars on track and sporting operations at race weekends.