Tom Stalker was unable to make it six medals for the British boxing team, losing his light-welterweight quarter-final to Munkh-Erdene Uranchimeg 23-22.
Stalker's camp lodged an appeal, but it was rejected by the International Amateur Boxing Association.
Uranchimeg appeared to have the better of the first round but team captain Stalker felt he did enough in the second and third to nick it.
Belfast light-flyweight Paddy Barnes secured Ireland's fourth boxing medal.
Luke Campbell, Fred Evans, Anthony Ogogo and Anthony Joshua have their semi-finals on Friday, while Nicola Adams goes on Thursday.
Commonwealth champion and third seed Stalker overcame a torrid encounter against India's Manoj Kumar to reach the last eight.
And Uranchimeg made the faster start on this occasion, his snap-hook particularly potent in the first round, although the judges scored it 6-6.
Stalker, 28, stepped things up in the second but was still shipping some heavy blows, before the final round exploded as both men scented victory.
Stalker poured on the pressure in the final three minutes, the ExCeL crowd responding to his efforts and drowning out a vociferous Mongolian contingent.
"It was a close fight, the Mongolian was a very good operator, but he [Stalker] gave it his best shot," said Stalker's coach Dave Alloway.
"He's devastated because he wanted to win a medal in his home Olympics. He's trained so hard for it, so to be one point away from a medal, one scoring blow, that's what hurts."
Uranchimeg will fight Denys Berinchyk in the last four after the Ukrainian world silver medallist beat Jeffrey Horn of Australia 21-13.
The other semi-final in the 64kg division will take place between Cuba's Roniel Iglesias, who beat Uzbekistan's Uktamjon Rahmonov 21-15, and Italy's Vincenzo Mangiacapre, who beat Daniyar Yeleussinov of Kazakhstan 16-12.
Barnes is the first Irish boxer to win medals in successive Games, having taken home bronze from Beijing four years ago.
In a high-quality first round, Barnes opened up a two-point lead over India's Devendro Laishram courtesy of some sharp jabs and flashing right hands.
Barnes, a former European champion, stretched his lead to seven points after the second and eventually won 23-18, having taken his foot off the gas in the third.
The 25-year-old faces three-time world and reigning Olympic champion Zou Shiming in the last four.
Zou beat Kazakhstan's Birzhan Zhakypov 13-10, although few in the arena would have complained if the result had gone the other way.
The other semi-final in the 49kg division will be between Russia's David Ayrapetyan and Thailand's Kaeo Pongprayoon, who has assured his country's third medal of the Games.
In the light-heavyweight competition, Brazil's Yamaguchi Falcao Florentino joined brother Esquiva, a middleweight, into the semi-finals with an 18-15 upset of Cuba's world champion Julio La Cruz Peraza.
Florentino will next fight Egor Mekhontcev after the Russian produced a rousing finish to beat Uzbekistan's Elshod Rasulov 19-15.
Kazakhstan's Adilbek Niyazymbetov overcame a second-round knockdown to beat Iran's Ehsan Rouzbahani 13-10 and will fight Oleksandr Gvozdyk next.
Gvozdyk beat Abdelhafid Benchabla of Algeria 19-17 but it was another questionable decision in a tournament of questionable decisions.
Benchabla received a warning for holding as the clock ticked down in the third round, which meant two points were added to Gvozdyk's total.