Australia and New Zealand sailed a victory lap in the medal race with the Olympic 49er gold and silver medals already sealed in Weymouth.
Australians Nathan Outteridge and Iain Jensen dominated the high-performance class with Kiwi training partners Peter Burling and Blair Tuke also a long way ahead of the pack.
Six crews went into the final with a chance of bronze, including British duo Stevie Morrison and Ben Rhodes, but Denmark's Allan Norregaard and Peter Lang hung on to third to claim bronze.
Morrison and Rhodes, who went to the 2008 Beijing Games as favourites but came ninth, crossed the line fifth in a very tense, light-wind medal race and ended fifth overall.
"It's pretty gutting, we threw it away in the middle of the week," said Morrison. "We should have been far better off in the points but that's how it is."
The Exmouth pair have been friends since childhood and were world champions in 2008 heading into the Olympics. They struggled with equipment and the light, shifty conditions and confess they took a long time to recover.
They came through a competitive selection battle and were looking promising with two straight wins halfway through the 15-race competition before falling back to fifth, six points off bronze, heading into the medal race.
The Britons got off to a decent start and chased Austria up the first leg but the Kiwis slipped through, as did Denmark and Australia on the final run to the finish.
Rhodes added: "It was always going to be tough. We came out with the aim of sailing a good race and we did a reasonable job, but we didn't quite get the hand we wanted.
"In China we weren't good enough, whereas here I think we were. It's done, it's gutting, but we've got an awful lot to be proud of.
"We're still good mates, still will be. You never know, maybe we'll come and try again."
Outteridge was denied gold four years ago when he and former partner Ben Austin capsized within 100m of the finish line to end fifth. After Beijing, he teamed up with old friend Jensen and the pair became three-time 49er world champions and were favourites heading into London 2012.
"It's amazing, we've sailed brilliantly for four years and this week was one of our best weeks so far," said Outteridge. "It's great it happened at the Olympics, it was awesome to enjoy it with the Kiwis.
"We saw [in Australia] what happened with Tom [Slingbsy] when he won. It's awesome for sailing in Australia. We'll enjoy the moment but the next campaign starts soon."