Tony Stewart's hopes of making the 2013 Chase for the NASCAR Sprint Cup Series championship have been put at serious risk after he broke his left on Monday night while competing in a race at Southern Iowa Speedway.

With five laps remaining, Stewart was leading the 30-lap race in a 360 winged sprint car when a lapped car spun ahead of him and collected Stewart and two others.

"It looked like he got into a lapped car," race winner Brian Brown told the Des Moines Register. "When I got close, he was flipping cage down. I didn't really have time to watch and see what was going on.

"First and foremost, we're concerned about Tony and making sure he's all right," he added. "Anytime you see him wreck like that and then leave in an ambulance, it's never good. Hopefully he's okay."

"By the time I got there, all I saw was carnage," said another competitor in the race, driver Tony Bruce Jr. immediately afterwards. "Cars and parts and pieces laying every which direction and it looked pretty bad. I knew it looked pretty bad, but hopefully Stewart is okay."

Eyewitnesses at the track said that the 42-year-old driver was conscious and talking after the accident as he was placed on a stretcher as he was transported to a local hospital. He has been diagnosed overnight with a broken right tibia and fibula. A further medical update will be made on Tuesday afternoon.

The Stewart-Haas Racing team, of which Stewart is a co-owner as well as lead driver, confirmed that Stewart will miss next weekend's NASCAR Cup race at Watkins Glen - where he's a five-time winner - although no replacement driver has yet been decided upon. The team also cancelled its scheduled Tuesday test at Atlanta Motor Speedway.

Stewart finished in ninth place in Sunday's NASCAR Cup race at Pocono Raceway as he seeks to make it into the top ten to qualify for the Chase play-offs that begin next month. Missing a race at this late stage with only five events remaining before the cut-off when things are so tight - he's currently in 11th place in the standings - puts his hopes of competing for the title in the autumn in serious jeopardy. He does however have one win in 2013 which may still see him squeak through on a 'most wins' wildcard.

Stewart is a big supporter of local and regional sprint car racing and his participation in events is a huge boost to attendances. Also competing in Monday night's race was NASCAR Nationwide Series regular Kyle Larson, who went on to finish second behind Brown after Stewart's accident.

"I didn't see how it happened, I just saw cars going everywhere," said Larson of Stewart's accident on Monday night. "I had to duck through the infield and luckily I missed it.

"Me and Tony do a lot of the same things, racing every night that we can," Larson added. "He grew up racing open-wheel dirt stuff like I've been doing. He's probably the best race car driver ever."

But even before Monday's accident, Stewart had been having a bumpy time of sprint car racing of late. On July 16 he inadvertently triggered a ten-car accident that left another competitor, 19-year-old Alysha Ruggles, with a broken vertebra in her back. And last week Stewart rolled his car five times during a race at Ohsweken Speedway in Ontario.

Asked whether he should be risking himself by competing at such events, the typically plain spoken Stewart had brushed aside questions from journalists at Pocono on Friday: "You mortals have got to learn, you guys need to watch more sprint car videos and stuff," he said. "It was not a big deal. It's starting to get annoying this week about that. That was just an average sprint car wreck. When they wreck, they get upside down like that."

Stewart's good friend and fellow NASCAR competitor Jason Leffler was killed in an accident at Bridgeport Speedway in Swedesboro, N.J. in June, and on Sunday veteran sprint car driver Kramer Williamson died after a crash in a qualifying race at Lincoln Speedway in central Pennsylvania.