Aug.28 (GMM) A rain shower slowed the action towards the end of the final 60-minute practice session on Saturday morning, but there was frantic activity inside the Red Bull garage.

With two hours to go before the crucial qualifying hour at Spa-Francorchamps, Sebastian Vettel's RB6 had groaned to a stop in the pitlane, requiring a push from the marshals into the team mechanics' custody.

Team boss Christian Horner immediately said he did not know the reason for the stoppage.

Earlier, it had emerged that German Vettel would tackle the rest of the Belgian grand prix weekend with a used Renault engine, while the other leading contenders had new engines fitted.

Before the late rain fell, Vettel's teammate and world championship leader Mark Webber - who nearly ran into the stricken sister car in the narrow pitlane - had set the fastest lap time.

After dominating on Friday, Ferrari's Fernando Alonso was sixth fastest on Saturday morning; slower than both Red Bulls and McLarens.

"We must be humble because we see this morning that the Red Bulls and McLaren are so strong," Ferrari boss Stefano Domenicali told the BBC.

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