Liam Tancock fell short of completing his medal collection when he was fifth in the Olympic 100 metres backstroke won in a new Olympic record by American Matt Grevers.
Tancock is double swimming world champion and world record holder in the 50m backstroke but that is not an Olympic event.
The 26-year-old's best result over two lengths had been world bronze in 2007 a year before he finished sixth at the Beijing Games.
However, he qualified third fastest into the final and there were hopes he could force his way on to the rostrum.
Third at the halfway mark, the Exeter-born swimmer fell just short as he touched in 53.35 seconds with Grevers' winning time 52.16.