Leading drivers say Sunday's Belgian Grand Prix should be called off if Friday's torrential rain persists.
Schumacher walks through the rain at Spa
Red Bull test driver Vitantonio Liuzzi crashed in the morning session and the second was washed out when heavy rain made the circuit too dangerous.
"In these conditions nobody can drive except maybe behind the safety car. A normal race is not possible," said world champion Michael Schumacher.
"If it is raining like that, we can't race on Sunday," said Jenson Button.
"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.
"The forecast is for rain on Sunday. It's OK 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," added the BAR star.
But Formula One boss Bernie Ecclestone played down speculation that the race might be cancelled.
"They've been racing here in these conditions for years. There'll be a race on Sunday," he said
Only a handful of laps were run in the first practice session on Friday, and the weather is predicted to get worse rather than improve over the weekend.
Schumacher added: "You could see when the drivers that went out were on track that they couldn't really drive."
Renault's Fernando Alonso can clinch the drivers' championship if he wins on Sunday and McLaren's Kimi Raikkonen does not make the podium.
But the Spaniard did not take to the track all day, partly because of the weather and partly because of a team strategy to spare his car's engine ahead of a potentially punishing race.