Honda will introduce a new specification of engine on Fernando Alonso's Formula 1 car for the Russian Grand Prix, meaning the Spaniard will have to take a grid penalty.
The Japanese manufacturer has spent its remaining four 2015 F1 engine development tokens on an internal combustion engine upgrade, plus associated exhaust parts.
Alonso will thus receive a new ICE, turbo, MGU-H and control electronics for this weekend's Sochi race.
As he has already used nine ICEs, eight turbos, eight MGU-Hs and six CEs this year, the double world champion is set to receive grid penalties amounting to 25 places, which will translate to a back-of-the-grid start.
Honda had hoped to introduce its new specification to Jenson Button's car too, but was unable to manufacture enough new parts in time to make another new unit for this grand prix.
Alonso has received the new engine first on account of racking up more miles than Button on his previous unit.
Button will continue to use the engine he ran in the previous race in Japan.
Honda's chief weakness concerns the efficiency of its MGU-H.
Solving this problem requires a fundamental change in engine architecture, which means the Japanese manufacturer feels it cannot act until the winter, when more development time and tokens will become available.
Speaking to AUTOSPORT before the latest upgrade was confirmed, Honda F1 chief Yasuhisa Arai said: "The logical step is to change the compressor area, but because of the layout that's not going to happen.
"We are still looking to improve the internal combustion side of the engine, but without changing the layout."
Honda has introduced its final 2015 update here in order to give it a better chance of strong performance at the United States GP at the end of the month, which is an important event for McLaren-Honda's sponsors.
"It isn't logical to bring the updates in a race where the circuit layout is already against us at a deployment situation," Arai added.
"Austin should be one of the better races for us, that suits the car, and we do have sponsors that have big presence in that race.
"Obviously tokens involved a big change, which involves a penalty. Therefore we will try not to take penalties at a race that's important for us."
Honda will thus still face a dilemma on Button's car, because changing his engine again before the end of the season will incur further grid penalties for the 2009 world champion.