British modern pentathlete Jamie Cooke finished fourth at the European Championships in Kent, achieving the 2012 Olympic qualification standard.
Cooke, 20, was 29th after the fencing stage but a world record 200m swim improved his standings, whilst team-mate Nick Woodbridge finished 26th.
Britain can have a maximum of two pentathletes per gender at London 2012.
Woodbridge and Sam Weale must now battle to better Cooke's fourth place finish to compete at the Games.
Cooke recorded a modern pentathlon swimming world record in the 200m freestyle of 1:49.59.
"This is the best day of my life," said Cooke, who entered the competition ranked 22nd in the senior world rankings and second in the junior rankings.
Cooke ended the competition on 5880 points and added: "It's probably the best competition I've ever done, I'm so happy.
"I thought it was going to be a bad day after the fencing, but I swam really fast and that took me back up. The horse I had was gorgeous and the combined just clicked.
"It's still going to be really tough. Sam and Nick will be pushing and pushing. They will bounce back. There's still a long way to go and I've got to put the hard work in.
Russia's Andrei Moiseev took the gold medal with 5996 points.
Fellow Brits Woodbridge and Weale have until June next year to outperform Cooke at another Olympic qualification event or in the world rankings to compete at the London Games.
British Pentathlon Performance Director Jan Bartu said: "We had a fantastic day today. Jamie produced an exceptional performance.
"He has worked hard and he's got good coaches and a good support team behind him.
"I'd particularly like to highlight contribution of Philipp Waeffler, the men's coach, who has done a cracking job nurturing Jamie and he has delivered for Britain."
The women's final begins on Sunday with Britain's 2008 Olympic silver medallist Heather Fell, Katy Burke, Freyja Prentice and Mhairi Spence all taking part.
Modern pentathlon comprises the sports of fencing, swimming, show jumping, running and shooting.
Each performance during the day is converted into pentathlon points and the winner is the athlete to amass the most points at the end of the day.