Eric Boullier believes the success of the Renault team's F-duct was the cause for the performance gain that allowed Robert Kubica to finish on the podium at Spa.

Kubica ran second to Hamilton after Vettel had taken out Jenson Button, but slipped to third during his last pitstop.

But Renault team boss Boullier was delighted with the result and the pace of the car, which was able to stay with the front runners throughout the race.

"I am pretty sure than half the performance gain we made today was down to the F-duct," he told reporters. "I have to pay tribute to the team because everybody was impressed that we fitted the cars with the F-duct and it worked straightaway - and was matching the simulation figures.

"The blown floor was clearly very challenging to make it work properly and it takes a couple of races to do it, and that is why it looks like we didn't improve much. The F-duct clearly was a bit step forward and now we have the same tools as everyone else on the grid."

Boullier reckons Renault can now reach Mercedes for fourth place in the constructors' championship.