Nico Rosberg has been forced to revert to the old specification of Mercedes engine following a problem in final practice for the Italian Grand Prix.
The German hit trouble with the updated power unit on Saturday morning, and following analysis of the issue after the session, it has been decided that he should go back to running the power unit from Belgium.
Mercedes confirmed the news on its Twitter feed.
It said: "Un-diagnosed engine problem for Nico Rosberg in FP3 means we're reverting back to the Spa race power unit on his #F1 W06 Hybrid for qualy.
"As this engine is already in the race pool, Nico Rosberg will not receive a penalty for taking the engine change."
Mercedes had elected to use all seven of its in-season development tokens for the new-spec engines at the Italian Grand Prix in a bid to fast track developments it intends to bring on board for 2016.
The effort to bring the updates for Monza had been immense, with the final power unit only arriving on Thursday lunchtime.
Mercedes F1 engine chief Andy Cowell said on Friday that it had been a tight call to green light the engine – especially because there were obvious reliability risks.
"There were some late night decisions on Wednesday as to what we are going to do here," he said. "It is a race, it is great collaboration between scientists, sharing information, coming up with ideas, doing experiments to confirm it, and making bits to build a multi cylinder.
"Then long running it, it has be broken, changing it, going back around and wanting to get to the earliest race."