A superb performance in the pool from the 20-year-old Briton, who holds the world swimming record in his sport, helped him turn around a disappointing opening discipline of fencing where he finished 29th.

Cooke powered his way up to seventh in the leaderboard with another phenomenal swim – his 200-metre freestyle time of 1min 53.56sec was almost seven seconds faster than the second quickest time.

The young Briton continued his momentum in the ride, doing enough to arrive at the concluding run/shoot phase with a shot at the medal.

Cooke went into the run/shoot in fifth place, 14 seconds behind leader Kim Soengjin of Korea in a run/shoot in which the top-10 starters were separated by just 30 seconds.

He was one of a group of five pentathletes that left the shooting range together at the end of the first shoot.

Cheered on by the other members of the GB team who were not competing, Cooke took the lead after the second of three shoots and crossed the finish line five seconds clear of Kazakhstan's Pavel Iliashenko to take gold with 5,880 points.

Iliashenko scored 5,860, with Alexander Nobis of Germany taking bronze with 5,856.

"This is an amazing achievement, just fantastic," said Cooke, who is originally from Cheltenham but now trains at the Pentathlon GB High Performance Centre at the University of Bath and lives in the city.

"I think this is my biggest achievement yet," added the athlete who has already achieved the London 2012 qualifying standard, but is not yet certain of place on the team.

"I didn't expect to win here today, especially after my fence, I just couldn't get a hit.

"My main goal at this event was to get experience for next year's senior tour.

"There was a lot of pressure on me in the run/shoot as there were some fantastic runners around me.

"I also thought I had something to prove after the European Juniors.

"I was second at the World Youths a few years ago and when I realised I had a chance of winning after the second shoot here today I told myself  there would be no way that I would be coming second."

Philipp Waeffler, men's head coach for Pentathlon GB, said: "It was a bit of a slow start, but after the fencing everything went really, really well.

"Jamie was strong mentally throughout the day and I'm very happy for him."

Cooke achieved the London 2012 qualifying standard by finishing fourth at the senior European Championships at Medway in Kent in August, but came 21st at the European Junior Championships in Poland in September, so is not yet certain of place on the team.