Sebastian Vettel used his only flying lap of the session to top the times in final practice for the Spanish Grand Prix in Barcelona.

The defending champion had spent much of the session in the garage as Red Bull worked on an engine issue on his car but was able to get out for one flying lap right at the end of the session.

Despite his lack of running, Vettel duly posted a time of 1min 21.707secs which was enough to beat team-mate Mark Webber to top spot by 0.084secs.

Webber had been quickest throughout the session, as was the case yesterday, and he finished up well over a second clear of Michael Schumacher's Mercedes, with the German best of the rest behind the dominant Red Bulls.

Lewis Hamilton was a fraction of a second further back in fourth ahead of McLaren team-mate Jenson Button and the second Mercedes of Nico Rosberg.

While the session was largely incident free, the one incident that did occur was a major one after the Renault of Nick Heidfeld caught fire just under half an hour into the one hour of running.

The German was able to bring the car to a halt on the side of the circuit with the rear well ablaze and escape uninjured, although the team now faces a battle to get him out for qualifying later today.

"It was one of the biggest fires I've had since I've been in the car," he told the BBC. "We just have to take the car apart, it is quite damaged on the right-hand side. If the chassis is damaged it's going to be a problem. It's too early to know. We have some ideas what caused the fire but I don't want to speculate."

Elsewhere, Ferrari was forced to swap back to an older spec rear wing after the part fitted for Friday practice was deemed illegal by the FIA, with Fernando Alonso and Felipe Massa down in ninth and eleventh - both reported to be unhappy with their set-up while Alonso also said over the radio that his car was 'undriveable' at points.

A number of drivers, including Rosberg and Narain Kartkikeyan ran wide through the final chicane while exploring the limits of the tyres.