(Updates with advice to stay away, quotes)
By Alan Baldwin
SILVERSTONE, England, July 6 (Reuters) - Tens of thousands of Formula One fans have been urged to stay away or risk being turned away from the British Grand Prix on Saturday following a day of traffic gridlock caused by the closure of waterlogged campsites.
In scenes similar to a 2000 fiasco at the Silverstone circuit, when heavy rain closed car parks and turned green fields into muddy quagmires, approach roads were jammed for hours on end on Friday.
Silverstone said they would refund unused tickets for Friday and Saturday and in a sign of mounting desperation asked anyone with public car parking passes to stay away on Saturday, qualifying day for the race, to ensure fields were not too churned up for Sunday.
The circuit's managing director Richard Phillips, looking tired and his voice showing the strain as the sun finally shone after steady rain, said Silverstone would not be able to cater for up to 30,000 of the expected crowd of 100,000 on Saturday.
"Our ultimate aim now is to try and preserve as much as we possibly can for Sunday," he told reporters, explaining half the public car parks had been lost to the weather and offering an "unreserved apology" to fans.
The race will go ahead as planned, however.
"What we can do tomorrow is accept people that are in Park and Ride (areas)...people that are in camping and that can walk in. We are strongly advising anybody with a public parking ticket not to come tomorrow."
Corporate guests, sponsors, media and others who park inside the circuit will not be affected.
Phillips said without people staying away there would otherwise come a point where incoming traffic would have to be directed straight past the circuit and on to exit roads.
A crowd of 80,000, more than turn up on race day at some other circuits, was expected at Friday's two practice sessions on what was forecast to be the wettest day of the British summer.
With many of the teams based locally, and with McLaren's two British world champions Jenson Button and Lewis Hamilton having a chance of winning, Sunday's likely attendance has been put at more than 125,000.
Phillips said the refunds offered would cost Silverstone "an awful lot of money' without giving a precise sum.
"I wasn't here in 2000 but this is my 2000 I guess, isn't it?," he said. "This (the race) is something I've been very proud of over the years and I feel I almost could cry now."
In 2000, when the grand prix was held in April, tractors had to pull cars out of deep ruts and public vehicles were banned altogether on the Saturday when car parks were closed.
On Friday cars were turned away from private campsites while, at the main official site, camper vans were towed in one at a time.
Some corporate guests had to abandon their transport to walk the last few miles into the circuit in the faint hope of seeing any action.
One small mercy was that Formula One supremo Bernie Ecclestone, a fierce critic in the past of a circuit owned by the British Racing Drivers' Club, had been persuaded not to travel up from London.
International Olympic Committee (IOC) president Jacques Rogge is due to be one of the 81-year-old billionaire's guests at Silverstone on Sunday less than three weeks before the start of the London Games.
Much has changed for the better since 2000, with the circuit spending more than one million pounds ($1.55 million) a year on preparing and managing car parks and trying to ensure the traffic flows smoothly.
An extra mile of filter drainage has been put in this year and 1,100 tonnes of hardcore added to car parks but the sheer number of people trying to get into campsites without pre-booking, coupled with the weather, has been hard to handle.
The British summer, always unpredictable, has been particularly damp after the wettest April in more than a century. The forecast is for rain throughout the weekend.
Camping is a big part of Silverstone, surrounded by fields in the heart of England, with many drivers staying in motorhomes on site rather than retreating to luxury hotels.
At its best, with the sun shining and the whiff of barbecue smoke in the air, it is one of the most inclusive races of any on the calendar...providing the fans can get in.
($1 = 0.6443 British pounds) (Additional reporting by Keith Weir editing by Alison Wildey)