The William Haggas-trained colt, ridden by Darryll Holland, was adjudged to have dead-heated with the filly Coupe De Champe in the Group Three contest.
The result was something of a surprise to racegoers as the French challenger appeared to the naked eye and on TV replays to have got up in the dying strides.
The photo-finish mechanism was found to be out of action, however, apparently because it had been struck by lightning on Sunday.
Brunel had established a clear lead in the seven-furlong race until Christophe Lemaire delivered what appeared to be a well-timed swoop down the outide.
Harry Herbert, racing manager for Brunel’s owners Highclere Thoroughbred Racing X, said: “I phoned the lady who was representing us there and she said it looked as if we were beaten.
“But then she phoned me back to say the photo-finish equipment had been knocked out by a lightning strike and that they’d had to declare a dead-heat.”
The other British runner at Longchamp, Vodafone Derby 11th Kings Quay, fared less well as he finished seventh of the eight runners in the Group Three Prix Daphnis.
He raced quite prominently early on under Dane O’Neill before fading in the straight.
The race went to the Jean-Claude Rouget-trained Pinson under Stephane Pasquier, who won by a length from Melanosporum.
nPromotion is 9-2 favourite with Ladbrokes to make amends for an unlucky run at the Royal Ascot meeting in Saturday’s Bet365 Old Newton Cup at Haydock.
The five-year-old, trained by Sir Michael Stoute, finished fifth behind stable companion Imperial Stride after being hampered in the early stages of the Wolferton Handicap at York.
Totesport, however, make Promotion one of three 13-2 co-favourites with Mark Johnston’s Quizzene and John Dunlop’s Tawqeet for the £75,000 heritage handicap, for which there were 30 entries.
Totesport spokesman Damian Walker said: “Promotion must go close if staying this mile-and-a-half trip, while Quizzene and Tawqeet will be dangers to all provided they can get into the race, as they need horses to come out.”
Bet365 Old Newton Cup betting:
Ladbrokes: 9-2 Promotion, 6-1 Flamboyant Lad, 9-1 Maraakeb, 10-1 Dunaskin, Tawqeet, 12-1 Bar Golden Quest, Grampian, Vaughan, 14-1 Big Moment, Camrose, Etesaal, Hambleden, Motive, Red Lancer, Zeitgeist, 16-1 Bar.