The Sir Michael Stoute-trained colt was placed in the Dante at York and the King Edward VII Stakes at Royal Ascot before he impressed in last month's Gordon Stakes at Goodwood.
Stoute's assistant James Horton said: "We were very pleased with Goodwood and we're very pleased with where he is now.
"He's still a big, weak, frame of a horse who we think is going to be a better horse next year.
"He won on soft ground at Goodwood, but stepping up against a better class of opposition, the better the ground the better for him."
Champion jockey Jim Crowley has been booked to partner the son of Sea The Stars for the first time in the final Classic of the season, for which he is the 7/2 joint-favourite at Sky Bet.
Irish Derby hero Capri, the other joint-favourite with the Leeds firm, looks set to lead a three-strong team into battle for Aidan O'Brien.
The Galileo colt appears the Ballydoyle handler's biggest hope, having been given a break since he defeated Cracksman at the Curragh at the start of July.
O'Brien has also left in Abyssinian, Air Supremacy, Douglas Macarthur, The Anvil and Venice Beach, but that squad is likely to be trimmed down at the final declaration stage on Thursday.
Coolmore UK representative Kevin Buckley told At The Races: "The St Leger has a very high standing at Ballydoyle - it's the final Classic and we want to win it.
"I'm getting the vibe we'll run three - Capri, Venice Beach and Douglas Macarthur."
O'Brien first won the Doncaster Classic with Milan in 2001 and since added to his tally with Brian Boru (2003), Scorpion (2005) and Leading Light (2013).
John Gosden, who has claimed the St Leger four times, is set to saddle Goodwood Cup winner Stradivarius and Coronet, the only filly in a potential field of 13 runners.
The grey daughter of Dubawi was last seen chasing home esteemed stablemate Enable in the Yorkshire Oaks.
Speaking at a St Leger press event, Gosden's wife Rachel Hood said: "The plan is to run both Stradivarius and Coronet. We're not worried about the ground for her (Coronet). I think she's very ready for the race."
Defoe has won on his last four starts for Roger Varian and is poised to step up to Group One level for the first time.
Raheen House is also a major contender, having won the Bahrain Trophy at Newmarket in July.
Trainer Brian Meehan said: "Raheen House is a knockout price at 12/1 but it's a massively open St Leger and his preparation has been good."
Andrew Balding's Count Octave and the Joseph O'Brien-trained Rekindling were also left in contention at the confirmation stage for the Leger.
The official going at Doncaster on Monday was soft, heavy in places, but clerk of the course Roderick Duncan is expecting predominantly dry weather in the second part of the week.
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