Horatio Nelson finished down the field in the 2,000 Guineas

The decision was announced after the colt had tests on Wednesday morning following a workout the previous day.

Meanwhile,Olympian Odyssey will bypass the race and head to the French equivalent at Chantilly 24 hours later.

The Barry Hills-trained colt was third in the 2,000 Guineas and owner Bill Gredley fears he will not have the stamina for the Derby trip.

Fallon was most impressed by Horatio Nelson after also putting O'Brien's other contenders - Septimus, Dylan Thomas and Mountain - through their paces.

Plans for Derrinstown Stud Derby Trial runner-up Mountain have yet to be finalised.

Horatio Nelson's price to win the Epsom Classic had plummeted after speculation Fallon, who also had the option of riding Coolmore's Papal Bull from Sir Michael Stoute's yard, held a preference for him.

Robert Winston will be aboard Papal Bull in his first Derby ride.

On Tuesday, bookmakers slashed Horatio Nelson's odds from about 8-1 to 4-1 following a series of sizeable bets ahead of the 227th running of the big race.

The colt finished eighth behind his Fallon-ridden stablemate George Washington in the Stan James 2,000 Guineas on his reappearance.

Fallon is bidding for his fourth English Derby win in eight years, after success on Oath (1999), Kris Kin (2003) and North Light (2004).