Only three cars left the pits in the course of the one-hour session – and one of those never made it back.

Vitantonio Liuzzi was the first man to dare to venture out on track and he soon paid the price for his bravery.

After tip-toeing through the ultra-fast Eau Rouge section, the Italian was caught out under braking for Les Combes.

The rear of his Red Bull snapped out of control on the sodden track, pitching him into one barrier and then another and sending a clear warning to the rest of the drivers.

Robert Doornbos was the only other man on track at the time and he slowly trundled back to the pits while world championship leader Fernando Alonso came out for one lap only because he missed first practice.

If race officials judge conditions to be too dangerous on Sunday they can order the race to be run behind a safety car for some or all of the 44 laps, when drivers would be banned from racing.

Formula One’s leading drivers have vowed to disappoint a near capacity crowd if the forecast of rain on Sunday rings true.

BAR-Honda star Jenson Button said: “If it is raining like that no, we can’t race on Sunday. It would be too wet, definitely.

“It is dangerous. You are not in control of the car in any way. If you hit a big puddle like Liuzzi did that’s it, it could be a massive accident. You are not in control and that’s not what we’re here to do.

“The forecast is for rain on Sunday. It’s alright if it stops raining because the water runs away but it’s when it is raining consistently and constantly it’s just impossible to run.”

Outgoing world champion Michael Schumacher, who had wanted rain to level the playing field and give him a better chance of victory, echoed Button’s fears.

Ferrari driver Schumacher feels a safety car race would be the only solution if the rain returns on race day.

“It is hard to tell you how difficult it is driving in these conditions,” he said. “We probably needed a propeller to make it through this amount of water.

“It is too much rain. In these conditions nobody can drive except maybe the safety car. Maybe we would do a race under the safety car, but a normal race in these conditions is not possible.

“It is not me to ask if there will be a race, but if conditions are like this then it is not possible apart from behind the safety car.

“As far as we know now it is predicted to get worse and at the moment they are predicting heavy rain on Sunday. Today was supposed to be the best in terms of rain.”

The practice downpour came as a huge disappointment for a bumper crowd, which is believed to be the largest ever at Spa for Friday practice.

Most stayed on valiantly despite the rain – and the general lack of covered grandstands at Spa – although some left early when it became clear no further running was likely.

Button sympathised with the sodden fans, adding: “It is disappointing for the fans but we can’t really help with that situation. We can’t stop it raining here.

“It’s been a pretty boring day for all of us, not being able to do any running.”