BBC News: First Flow beats Politologue in Clarence House Chase at Ascot
Favourite Politologue, ridden by Harry Cobden, took the early lead and looked comfortable out front.
But 14-1 shot First Flow, under jockey David Bass, caught him and after making an early charge for the line went on to win by seven lengths.
"Down the hill, I didn't plan on taking it on that early but he just winged the fences," Bass told ITV.
"I'm so pleased for this horse. He's a real character. I can't really believe it."
The race marked First Flow - Kim Bailey's nine-year-old - stepping up to the top level over fences for the first time in his career.
Waiting Patiently was third, while Fanion D'Estruval came in fourth in a race also featuring Defi Du Seuil.