Overnight in the McLaren pits, the British team have removed an update package - including the Red Bull-style blown diffuser - from the MP4-25 car.
Red Bull’s Mark Webber suspected the championship leaders of "sandbagging" in Friday practice at Silverstone, but in fact McLaren was not happy with the improvements.
"It hasn’t been perfect, that’s the truth," said Jenson Button after the afternoon session, when he was 13th fastest.
Lewis Hamilton, although half a second quicker than the sister car, added: "The feeling was positive first thing but the changes we made didn’t make it better and that happens sometimes.
"It’s not something you can just bolt on," added the world championship leader.