Flyweight Andrew Selby secured his place at the 2012 Olympics after Khalid Yafai failed to make the weight for the second bout of their "box-off" by 300g.
Welshman Selby, 22, had won the first bout of a three-fight contest by 26 points to 12 at the British Amateur Boxing Championships on Friday.
Birmingham's Yafai was then unable to make the weight for Saturday's London meeting, handing Selby a 2-0 victory.
"I'm over the moon, it still hasn't hit me yet," Selby told BBC Sport.
"It'll hit me when I get home."
Selby won a silver medal at the World Championships in Azerbaijan last month but Yafai also made it to the quarter-finals, meaning both men had qualified for a place in London with only one spot available, prompting GB Boxing to schedule the three-fight decider.
"It is very unfortunate that the contest has had to be settled in this way because they are two great boxers and both deserved to go to the Olympics," said Rob McCracken, the British Amateur Boxing Association's performance director.
"It is a great achievement by Andrew, who has boxed superbly this year, but at the same time I am very disappointed for Khalid."
Selby had lost his two previous fights against Yafai but took the initiative early in Friday's encounter at York Hall when he caught his rival off-guard with a punch that sent his fellow 22-year-old stumbling to the canvas.
The Welshman held on for a comfortable victory but Yafai picked up a hand injury in the fight that hindered his effort to make the weight on Saturday.
"We normally have an optimal boxing weight but I knew I had to be strong for this fight and put an extra bit of weight on for an extra bit of power," Yafai told BBC Sport.
"I hurt my hand in the (first) fight and my hand swelled up and I had a bruised knuckle. I spent a lot of time getting that seen to by doctors and physios and by the time I got back to the hotel I was 1.7kg over.
"I tried to get most of it off last night and I did, and then I had 0.9 to get off this morning. We did train this morning for an hour and a half, we had two attempts this morning (at making the weight) but the scales closed at half past nine, and I lost out by 300g.
"I just want to say well done to Andrew, I wish him all the best and I hope he gets a gold for Great Britain."
The GB Championships pit the top amateur fighters from Scotland, England and Wales against members of GB Boxing's Olympic squad.
Tom Stalker, Anthony Joshua, Luke Campbell, Fred Evans and Andrew Selby are the GB boxers to have qualified for the Olympics so far. The second qualifying event for the men is in April, while the women's qualifying event is in China in May.
Amanda Coulson laid down a marker ahead of that crucial date in China with a 22-17 victory over fellow GB squad member Chantelle Cameron in the women's lightweight category.