A brilliant final clear round from Tina Cook saw Great Britain win a team silver medal behind Germany in the eventing competition at Greenwich Park.
Their brave gold medal challenge ultimately came up short as European champions and Olympic title favourites Germany took gold, with New Zealand clinching bronze.
Cook's show-jumping round - she had just one time fault - meant Britain secured second place and gave Britain its 18th Olympic post-war eventing medal alongside her team-mates of Zara Phillips, Nicola Wilson, William Fox-Pitt and Mary King.
After New Zealand's Mark Todd had one fence down it meant Cook, a double Olympic bronze medallist four years ago, could afford to have one fence down and still secure silver for her country.
And she collected just one time fault, meaning she had one of three counting scores alongside King, who jumped clear, and the Queen's granddaughter Phillips, who had seven faults.
Cook's performance on Miners Frolic was a remarkable feat of composure and accuracy under the most suffocating pressure, even if she did collect a time fault.
She called on all her previous top-level experience to give Britain the round they required.
Just over a year ago, Miners Frolic was fighting for his life, having contracted colitis. His condition was so bad, the vets did not think he would pull through.
He subsequently missed all of last year's Olympic qualifying competitions, and only secured the result he needed five months ago.
Cook said of her performance: "It was very much mind over matter.
"I was just focused about what I had been working on, focused on the course, desperately trying not to get a time fault.
"I did get my time fault, which I was really frustrated about.
"Team silver is in the bag, which is absolutely brilliant."
Phillips, making her Olympic debut on High Kingdom, was watched from the stands by her mother the Princess Royal, the Duke and Duchess of Cambridge, Prince Harry and the Duchess of Cornwall.
But she was disappointed with her round, which saw her knock the second fence down and also have three time faults.
Speaking immediately after the round, Phillips said: "I messed up, and I had to get on with it."
Britain's eventers had not won gold since Munich in 1972, when Phillips' father Captain Mark Phillips was in the team, but they gave it everything against a crack German quintet of world champion Michael Jung, Ingrid Klimke, Peter Thomsen, Dirk Schrade and Sandra Auffarth.
Attention now turns to the individual competition this afternoon, which could bring further British success.
King, seeking her first Olympic individual medal, lies third, just ahead of Cook.
Sweden's Sara Algotsson Ostholt leads the individual competition, with Jung second. Jung is bidding to become the first rider in eventing history to hold Olympic, world and European titles simultaneously.
