Marsha pipped Lady Aurelia to the post to win the Nunthorpe Stakes on day three of the Ebor Festival at York.
Lady Aurelia, the Frankie Dettori-ridden favourite, led just strides from the line but Luke Morris closed aboard Sir Mark Prescott’s 8-1 shot to win in a photo finish, with Cotai Glory third.
Dettori celebrated as he crossed the line, but Marsha was awarded victory.
“When I saw Frankie put his arm up, I was gutted,” said Morris. “Then someone congratulated me.”
He added: “Sir Mark is a genius. I have been with him five years now and we’ve had some great days. It’s fantastic for the owners.”
Battaash, expected to be Lady Aurelia’s main challenger, finished fourth.
Lady Aurelia trainer Wes Ward was proud of his horse in defeat.
“She’s a true champion, these things happen. She ran a brilliant race and the other filly ran us down,” he said. “I think we got beat fair and square. She’s run a fantastic race and I’m very proud of her.”
Cornelius Lysaght, BBC 5 live racing correspondent
A dramatic result on a dramatic day for close finishes; Frankie Dettori was most certainly not the only one who thought he and Lady Aurelia had won.
But as the jockey saluted the crowd, many of whom – including me – were convinced he was correct, judge Di Clark was studying the photo and her verdict was Marsha by a nose.
Dettori declared himself “speechless”.
Tight finishes at York are notoriously difficult to be sure about, and most thought the Queen’s horse Dartmouth had beaten Montaly in the Lonsdale Cup earlier in the day, but the photo showed the reverse.