Iain Percy and Andrew Simpson were denied a second successive Olympic gold as Sweden won the final medal race to clinch the Star title in Weymouth.
The British pair led going into the final but an eighth-placed finish meant silver as Sweden's Fredrik Loof and Max Salminen won the race and title.
Brazil's Robert Scheidt and Bruno Prada slipped from silver to bronze after coming seventh in a tense finale.
GB finished on 34 points, two behind Sweden, with Brazil on 40 points.
Percy and Simpson, who won gold in Beijing in 2008, had taken an eight-point cushion over Brazil and a 12-point lead over Sweden into the double points-scoring medal race but were unable to keep pace with the Swedes in the light breeze.
Percy, 36, told BBC Sport: "It feels cruel sometimes. It was ridiculous conditions at the end. We feel a little robbed, but that is the way it goes.
"We're hurting so much inside but it makes it so much easier to know [the supporters] are so happy for us.
"The crowd for me today is going to be a highlight of my life and probably the only thing that puts a smile back on my face."
"What a difficult finish. The boats on the left-hand side of the field got a puff of wind and they all crossed the line together. The boys will be devastated. All week they've led this, most of the race they've been ahead, then just at the end their medal dreams disappeared."
Simpson, 36, added: "You are never 100% confident. We didn't get it right and can't blame anyone but ourselves. We had a great regatta overall.
"We are really frustrated. We tried our hardest, but it wasn't to be."
The Swedish boat took an early lead on the flat waters in front of the massed spectators around the Nothe Fort in Weymouth.
They were briefly overtaken by New Zealand at the third mark but they edged in front again as the battle intensified behind.
Britain were fifth at the final windward mark but lost places on the downwind leg to the finish.
Percy won gold in the Finn class in Sydney in 2000 but switched to the Star and finished fourth with crew Steve Mitchell four years later.
For the Beijing Games, he teamed up with childhood friend Simpson, who was making his Olympic debut, and the pair fought back from a slow start to the regatta to win gold, with Swedish skipper Loof taking bronze behind the Britons and the Brazilians.