"He was 6 years old when I first met him," said Dario Franchitti after the news was confirmed that Dan Wheldon had died of the injuries he sustained in an accident during the IndyCar Series finale in Las Vegas. "He was this little kid and the next thing you know he was my teammate.

"Right now I'm numb and speechless," he finished. "One minute you're joking around in driver intros and the next he's gone."

Wheldon's fellow IndyCar competitor Graham Rahal, seen on pit road clearly shaken after the drivers were informed of Wheldon's passing, tweeted: "My heart goes out to the Wheldon family. Dan was an incredible champion, great friend, great guy and someone to look up to ... Today we lost one of the greatest members of our family."

Penske driver Ryan Briscoe admitted that "I don't have words for the tragic loss of Dan today. He was a good man and will be missed by all of us."

"This is one of the saddest days of my life," said Alex Lloyd, another of those who had been out on track when the accident unfolded. "Can't believe we have lost him. He was a true champion and a great friend."

"I have seen many fine men/racers leave us prematurely over my lifetime," tweeted motor racing legend Emmerson Fittipaldi. "It is an unfortunate part of our sport. Godspeed Dan Wheldon."

A statement from Andretti Autosport - with whom Dan Wheldon won his first Indianapolis 500 in 2005 - paid tribute to his well-loved personality: "Dan brought such enthusiasm and passion to the sport not often seen in motorsports. We will remember Dan's tremendous racing accomplishments with our team as well his infectious personality," the team said. "We would like to express our deepest sympathy to Dan's family, racing team and friends today. Dan is one of IndyCar's greatest champions."

Indianapolis Motor Speedway president Jeff Belskus also paid tribute to the two-time Indy 500 champion: "We are incredibly saddened at the passing of Dan Wheldon. He was a great champion of the Indianapolis 500 and a wonderful ambassador for the race

"Most importantly, he was a fantastic husband, father and man - a good friend to so many in this sport. His memory will live forever at the Speedway, both through the magnitude of his accomplishments on the track and his magnetism off the track. Our deepest sympathies are extended to his entire family, team and fans."

"Rest in Peace Dan," added F1 driver Mark Webber on Twitter. "I remember our early days in the UK 95/96. Miss ya."

McLaren test driver Gary Paffett called the news, "A massive loss to the Motorsport world. He was a star and a real nice guy."

A former F1 driver, Mark Blundell, tweeted: "Days like this are never easy for any driver old and young, brings home the risk and underlines in bold, made tougher when you know the guy."

From NASCAR, reigning champion Jimmie Johnson said, "My thoughts and prayers go out to the Wheldon family... My heart hurts for all of the IRL community."

Kyle Busch also tweeted: "The loss of Dan Wheldon affects the whole racing community. May all our thoughts and prayers be with his family. He will be missed!"

Matt Neal - crowned BTCC champion just hours earlier - also posted his tribute to Wheldon, saying "Dan Wheldon RIP so sorry to hear, absolutely tragic," as did Gordon Sheddon: "Shocking news about Dan Wheldon! Brings life into perspective."

And Tomas Scheckter - who was competing in the race for Sarah Fisher Racing and who was involved in the aftermath of the horrific 15-car accident - summed up the feelings of many when he tweeted: "Lay in a dark room hope I wake up from the nightmare. This is not fair and not right it can't be true."

As a result of the accident - which put Will Power out of the race - Dario Franchitti defaulted into winning his third consecutive series title. But he was the first to say how little that meant to him right now.

"We put so much pressure on ourselves to win races and championships and today it doesn't matter," he said.