(Updates with Ecclestone, Williams quotes)
By Alan Baldwin
SILVERSTONE, England, June 29 (Reuters) - Champagne glasses clinked and tributes flowed on Saturday as Formula One champions past and present celebrated 600 grands prix for the Williams team.
A pensive Nigel Mansell watched as a screen showed old footage of his 1986 title hopes unravelling in a high-speed tyre blowout in Adelaide while Alain Prost - champion that year for McLaren and with Williams in 1993 - stood nearby.
"We wouldn't have a Formula One without Frank Williams," the sport's commercial supremo Bernie Ecclestone told Reuters. "He's part of the structure. He's still with us, fighting on as usual."
For all the past wins and titles, just one point in Sunday's British Grand Prix might be a cause for just as much celebration.
The once dominant team, winners of nine constructors' championships between 1980 and 1997, have not scored in the last eight races and are stuck in their worst ever start to a season.
Venezuelan Pastor Maldonado's against-the-odds victory in Spain last season was their first in nearly eight years and the team has only gone backwards since. It has been a decade since Williams came close to winning a constructors' championship.
With their cars 16th and 17th on the grid for Sunday's race, there was no immediate sign of the gloom lifting but Ecclestone was supportive.
"I knew Frank when he was struggling at the beginning. He'll survive. And start winning," said the Briton.
Even if a win for Williams on Sunday would be a bigger shock of the British sporting summer than Rafael Nadal's first-round exit at Wimbledon, Silverstone is full of positive energy for a team proud of its past but very much focused on the future.
"We only think maybe about yesterday about our mistakes and what we can learn, we think mainly about today and above all about the next win," Williams, who founded the team with Patrick Head in 1977, told the assembled guests.
"I can assure you that 'Team Willy' and the people here are really dedicated to success."
It will be a long and hard road back but there is still huge goodwill in the sport towards a fiercely independent and proudly British team who rank as the second most successful after Ferrari in terms of constructors' titles.
They have 114 wins, 127 poles and seven drivers' championships to their credit with the likes of Alan Jones, Keke Rosberg, Prost, Mansell, Nelson Piquet, Damon Hill and Jacques Villeneuve.
They have also lived through dark days, the 1986 road accident that left Williams paralysed and wheelchair-bound and the tragedy of Brazilian triple champion Ayrton Senna's death in one of their cars at Imola in 1994.
"Frank and (co-founder) Patrick (Head) are both legends. I raced for them in 2000, they gave me the opportunity to race in Formula One," said 2009 world champion Jenson Button, who began his career at Williams when they were in partnership with BMW.
"I've got great memories with them and with the whole team...it's a true racing team and that's what I love about it.
"With belief and determination he's achieved, or the team has achieved, so much in their career," added the Briton. "Six hundred grands prix. It really is phenomenal. Fair play and hopefully they will be here for many many more years."
In fact, the 600th race will be in Germany next week - with the team counting from the foundation of Williams Grand Prix Engineering in 1977 with a staff of 17 working out of an old carpet warehouse in Didcot - but Silverstone is home.
Flush with Saudi cash, the team took their first race win there in 1979 with Swiss driver Clay Regazzoni and celebrated their 100th win at the former airfield in 1997 with Canadian Villeneuve.
Since their foundation, 78 Formula One teams have come or gone or changed ownership but Williams are still battling on - even if the cars currently coming out of the factory are far less successful than the ones in the museum inside and sponsorship is harder to find.
In Germany, the FW35 cars will have the number 600 on the sidepods while the livery will also carry the names of the group's 691 employees.
"This weekend we will be rightly celebrating our heritage and reflecting on some of our defining moments over the past 36 years," said Claire Williams, Frank's daughter and now the deputy principal.
"This is also an opportunity for us to take stock and look ahead to the future, making sure that we have the necessary pieces in place to make our next 600 races just as memorable." (Editing by Ed Osmond and John Mehaffey)