DONCASTER, England (AP) — Camelot is odds-on favourite to become the first horse in 42 years to complete the English Triple Crown today when the unbeaten colt contests the St Leger, the world's oldest classic.

Nijinsky, ridden by Lester Piggott in 1970, was the last horse to win the 2,000 Guineas, the English Derby and the St Leger in one season, earning him a place in racing folklore.

History is clearly against the three-year-old Camelot, who is stepping up in the distance to nearly two miles (3,200 metres) for the last classic of 2012, but the form guide certainly isn't.

Raced sparingly, Aidan O'Brien's horse has won all five of his races without ever coming under serious pressure and British bookmakers have him down as the 2-5 favourite for the Doncaster race.

"He is built more like a miler — he is round and strong, not lean and angular. That is a little thing that would be in your mind," O'Brien said. "You are going to pull him way beyond his comfort zone.

"Those extreme distances, if you are not up to it, can break the heart. It tells you how much of a test it can be but Camelot is a brave horse and usually comes home."

A victory for Camelot would complete a wonderful season for O'Brien, who is bidding to become the first trainer to win all five British classics in a season.

"It's great to think that maybe we could go back and have the same feeling that those people had when Nijinsky was around," said O'Brien, whose 19-year-old son, Joseph, will be Camelot's jockey.

"He's different this horse. He looks different, everything about him is different. He's flesh and blood but there's a vibe around this horse. There's a bigger aura around him, and there has been from day one."

Camelot will have eight rivals today, with Main Sequence — the runner-up in the Derby — and Thought Worthy both at 8-1 in the ante-post betting.

Fifteen horses in total have won the Triple Crown. Nine have won the 2,000 Guineas and the Derby but gone on to lose the St Leger.