Barry Hills' colt took the pounds 45,700 prize in a hard-fought finish in testing ground. Pat Eddery sent his mount to the front five furlongs out after a slow early pace. Moonax quickened two lengths clear turning into the straight but looked beaten when headed by Always Earnest.
However, the 24-10 chance responded superbly to Eddery's hard driving and regained the lead in the final strides to win by a short-neck. Eddery said: 'He's very brave, and a great horse to ride, although once you get off him he'll kick you at the first opportunity]' The horse will stay in training next year.
Kithanga, the other cross- Channel challenger, dropped away tamely in the straight to finish sixth of seven.
Millyant had earlier initiated a British Group race double by winning the Prix du Petit Couvert over five furlongs. The Rae Guest-trained filly beat Princely Hush with Imperial Bailiwick in third.
Next Saturday, Barton Bank, last year's King George VI Chase winner, is set for a comeback in the Charlie Hall Chase at Wetherby. He has not raced since bursting a blood vessel early in 1994.
David Nicholson, who trains the gelding, said: 'There's been no recurrence of his blood vessel problem. I'd like to think he is still improving. We hope to get some rain but he runs whatever the ground - within reason.'
PRIX ROYAL-OAK (Group 1, 1m 7f Longchamp): 1. MOONAX (Pat Eddery); 2. Always Earnest (Alain Badel); 3. Dalara (Gerald Mosse). 7 ran. won by a short neck, two lengths. (trained by Barry Hills at Lambourn for Sheikh Mohammed).
Pari-Mutuel (one- franc stake): win 3.40; places 1.80, 2.10. Straight Forecast: 19.20.
PRIX DU PETIT COUVERT (Group 3, 5f Longchamp): 1. MILLYANT (C Asmussen); 2.
Princely Hush (M Fenton); 3. Imperial Bailiwick (J Williams). 6 ran. 3/4 , 1 1/2 . (R Guest, Newmarket, for C Mills). Pari-Mutuel: 3.60; 2.30 1.80. SF: 17.60.Reuse content