F-duct decision was wrong - Hamilton (UK only)
Former world champion Lewis Hamilton said the decision to remove the F-duct from his McLaren for the Italian Grand Prix was "the wrong choice".
Hamilton, who leads the drivers' standings, could only qualify fifth for Sunday's race at Monza.
"I'm disappointed. The car was sliding everywhere. We didn't have the downforce we need," he told BBC Sport.
But team boss Martin Whitmarsh said Hamilton had been held up by running too close to Mark Webber's Red Bull.
"I don't think it's a set-up issue," he commented. "If you put his best sector times together, he would be two tenths quicker.
"I think we put him out too close to Mark Webber, and if you're too close you can wash out [due to understeer] in high speed, like at Parabolica."
McLaren's Lewis Hamilton felt the small rear wing gave insufficient speed
The decision to run Hamilton without the F-duct, and with a low downforce set-up, was made after tests in free practice showed that it made virtually no difference to lap times.
However, team-mate Jenson Button chose to use the F-duct, which reduces drag on the straights at most circuits.
And following qualifying, Hamilton's comments suggested he believed the car in fact worked better at Monza with a high-downforce set-up, thus improving its stability under braking.
"Potentially it [not running the F-duct] had quite a big impact on the result," said 25-year-old Hamilton.
Whitmarsh defends Hamilton car set-up (UK only)
"We chose to go with the lighter downforce level and I'm quite disappointed with fifth place. I struggled a lot on each tyre [prime and the softer option tyres].
"It was a bit of a mistake but we'll push hard tomorrow."
Despite Hamilton's grid position, Whitmarsh backed him to make the best of the situation in the race.
"We've got the unusual situation where we've got the quickest and the slowest cars on the straight in our team," he said.
"I think Lewis can use that straight-line speed very aggressively. He wants to attack and he's certainly got an attacking set-up, so let's hope he can use it."
But Hamilton was not so confident. "Turn one is not easy, and it's going to be very tricky out there on the track trying to gain position," he said.
"There are long straights, but you've got to get close to the car in front, and that's going to be very hard because I can't follow through the corners, so it will be very tough tomorrow."