Welsh swimmer David Davies will not have the chance to improve on the Olympic open water silver medal he won in Beijing at the 2012 Games in London.
The British Olympic Association has confirmed rival Daniel Fogg will compete for Team GB in the 10km event.
Both Davies and Fogg had already been selected to compete at the Olympics in the pool in the 1500m but the open water is Davies's preferred event.
Davies accepted his 10k chances were "all over" after his display in trials.
The 27-year-old, who is expected to retire after the Olympics, needed a top-10 finish or to beat Englishman Fogg at last week's FINA Olympic Marathon Swim qualifier.
But the Barry swimmer finished 38th in Portugal and Fogg 35th, meaning Fogg will compete at London 2012 in Hyde Park on 10 August.
"I am really happy to have been selected for my second event at the Games," said 24-year-old Fogg, who finished 16th in the open water at the 2011 World Championships.
"I'm back into hard training after recovering from the FINA Open Water event in Portugal and have been focusing on my first event, the 1500m.
"I'll now sit down with my coach to look at the strategy for the open water event too, which is a week after the pool swim. It's fantastic to get to race in both events."
Davies, who was close to winning open water gold in 2008, admitted that Fogg, England's 1500m Commonwealth Games bronze medallist in Delhi two years ago, deserved the nod.