The 18-year-old raced a sizzling final 50m to touch in 3:43.36 and overhaul Australian Jack McLoughlin (3:43.52) and US swimmer Kieran Smith (3:43.94), who took bronze.

It was a major breakthrough for the young gun, who has little pedigree on the world stage.

“I just can’t believe that, it’s amazing. I felt better in the water this morning than yesterday and that’s it. I’m the Olympic champion now,” he said.

“I just put my head in the water and that’s it. I just can’t believe it. It’s a dream come true.”

Elijah Winnington, who shocked Mack Horton at the Australian trials to deny his countryman the chance of defending his Olympic title, led through 100m.

But McLoughlin hauled him in by the 200m mark and looked destined for gold until Hafnaoui’s burst of electric speed as he zeroed in on the finish line.