The 18-year-old Oxford rider Bradley Smith scored his first ever grand prix win in his 50th race in the 125cc event at Jerez.
Smith, who had qualified second fastest, streaked into the lead from the grid on his Bancaja Aspar Aprilia, and never relinquished his advantage.
His job was made easier when his two main rivals, team-mate Julian Simon and Italian Andrea Iannone (Aprilia) both crashed.
Championship leader Iannone went down on the opening lap, and Simon dived off several laps later when he was already 1.8s behind Smith.
Smith built up an extraordinary lead of nearly 20 seconds at one stage, in a class where slipstreaming and massed battles are more common. He eventually won by 13.524s.
Scott Redding, the 16-year-old who won his first grand prix at Donington Park last year, started the race in second place on his Blusens Aprilia.
He later dropped to eighth, but mounted a fresh charge for the podium that was helped when the German Juan Folger crashed.
On the last lap Redding ran wide on a corner, and had to settle for fourth place behind runner-up Sergio Gadea (Aprilia) and Marc Marquez (KTM). Danny Webb (Degraaf Aprilia) made it three Britons in the top eight.
1. Smith (Aprilia)
2. Gadea (Aprilia) +13.524sec
3. Marquez (KTM) +13.553sec
4. Redding (Aprilia) +14.251sec
5. Vazquez (Derbi) +14.758sec
6. Cortese (Derbi) +15.545sec
7. Espargaro (Derbi) +15.556sec
8. Webb (Aprilia) +17.772sec
9. Aegerter (Derbi) +18.960sec
10. Terol (Aprilia) +19.053sec