Dartmouth, ridden by Olivier Peslier for trainer Sir Michael Stoute, won June's Hardwicke Stakes at Royal Ascot.
Earlier, second favourite Hawkbill was ruled out of the £1.15m race.
The Coral-Eclipse winner is likely to run instead in next month's Juddmonte International at York.
Stoute will seek a record sixth victory in the King George, with Frenchman Peslier lined up to keep the ride.
The Queen has won the Group One race named after her parents only once before, when Aureole triumphed in 1954.
Hawkbill, owned by Sheikh Mohammed's Godolphin team, was about a 6-1 chance for Saturday's race.
Trainer Charlie Appleby said the horse was in fine form at his Newmarket base.
"We have decided he will miss the King George and be saved for races over a mile and a quarter," Appleby said.
"The Juddmonte is the likely next target."
Last year's King George winner Postponed is the odds-on favourite for a second win in Flat racing's mid-season showpiece over a mile and a half.
Postponed was trained by Luca Cumani 12 months ago before being switched to Roger Varian, and won the Coronation Cup at Epsom in June.
The Queen is set to have a runner in Saturday's King George VI & Queen Elizabeth Stakes at Ascot after her colt Dartmouth was added to the line-up at a cost of £75,000.