Ahead of today's sole practice session the air temperature is 20 degrees C, while the track temperature is 21 degrees. It is overcast with the threat of rain this morning and for much of the remainder of the weekend.

While Red Bull enjoyed a totally dominating Friday, attention overnight has focussed on the tyre failures suffered by Sebastian Vettel and Fernando Alonso. However, following an investigation by Pirelli it has been revealed that the damage was done by "debris". However, drivers remain concerned at the issue, particularly on such a demanding, high speed circuit.

The Bulls enjoyed a clear advantage yesterday - though Alonso was quickest in the wet morning session - enjoying an 08s advantage over third placed Romain Grosjean.

Tyre options are hard (prime) and medium (option), while there are two DRS zone, the first on the short pit straight and the second on the Kemmel Straight leading up the hill to Les Combes. Detection points are before the chicane and the other on the run down to Eau Rouge.

While Lotus, Ferrari and Force India looked strong, Mercedes looked at sixes and sevens, Hamilton, in particular, struggling for grip.

The lights go green and Bianchi first out, followed by Grosjean, Gutierrez and Chilton. More and more drivers pour onto the track all sporting the harder prime rubber.

Having only run it in yesterday's wet session, surprisingly Lotus has opted not to run its passive DRS for the remainder of the weekend. Sauber has yet to make a decision on it version.

The Williams duo are the first to post times, Bottas crossing the line at 1:51.540. Moments later Perez stops the clock at 50.548.

As Hamilton begins his first flying lap of the day, Bottas reclaims the top spot with a 49.806, just ahead of Button (50.088). Yesterday it was Les Combes that caught out Hamilton today it is the chicane, he misses it and subsequently crosses the line at 53.863. Elsewhere, a lock-up and detour for di Resta at La Source.

Indeed drivers are struggling on various parts of the track, Hulkenberg getting it all wrong and running off track after Rivage.

A 49.759 sees Raikkonen go quickest, the Finn one of the few drivers not running a new engine this week on one of the most demanding of circuits.

Despite the fact that he is still unhappy with the handling of his Mercedes, Hamilton improves to third with a 49.857. Elsewhere, former teammate Button is out of his car, the McLaren thought to be suffering a suspension problem.

The cameras pick up something flying off Raikkonen's car as he comes out of the chicane.

Massa, who yesterday had oversteer, is now suffering understeer, the Brazilian runs wide at T9 which has already caused problems for di Resta and Hulkenberg. It's a fast, downhill left-hander that leads to the fabulous Pouhon.