Great Britain won the team silver medal in the eventing competition at Greenwich Park today.
Their gold medal challenge ultimately came up short as European champions and Olympic title favourites Germany took gold, with New Zealand clinching bronze.
Tina Cook's showjumping 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 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.
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.
"He [High Kingdon] is a good jumper but he couldn't get out of where I put him, even the best jumpers can't do that.
"It was my fault not his. I didn't knock any others down and then I was a bit slow but after that he jumped fantastic and I'm really proud of him."
Phillips, the Queen's granddaughter, said the last three days had been "an emotional rollercoaster" but that the atmosphere of the 23,000 capacity crowd - in Diamond Jubilee year - had boosted the British team.
She added: "It's incredible, a massive lift when you come into the arena, massive support, and I'm really grateful to everyone who has come and cheered for us, all week it has been the same.
"I definitely think it is a help, the pressure is what you put on yourself trying to get the best score for the team."
Phillips said she was delighted with how High Kingdom had responded.
She added: "I was really happy that he came out and jumped pretty well - he lost both his front shoes yesterday so to come out and do anything for me was pretty incredible and I am really happy with him and glad he has been able to do that today.
"For inexperienced horses it was a big ask and he's come up and given me everything. He's only going to get better."
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.
Cook was relieved not to let her team down.
"To be honest [I was] just trying to focus on jumping a clear round," she told BBC1. "The team has done so well and it was so close, we could have got silver or nothing, and that would have been awful.
"Gold would have been brilliant, silver is fantastic."
For Cook, the achievement was all the more emotional given Miners Frolic almost died after suffering from colitis last year.
"It's almost a fairytale," she said. "What can I say? It's [down] to the vets that kept him alive and the team at home that we got him here.
"It has been very stressful, a lot of grey hair and wrinkles. When it comes to this, and the crowd, it's unbelievable."
Cook quickly turned her attention to claiming a podium place in the individual event.
"I got one time fault, [which was] frustrating [as] it has dropped me an individual place," she said. "He jumped it clear and we got team silver but I've got to do better next time.
"We've got to go back in there and do it again and try to get an individual medal. That's the one I really want."
King, who won Olympic team medals in 2004 and 2008, once again came up trumps when it mattered.
She punched their air in delight after jumping the last fence on Imperial Cavalier, with her performance following a blistering clear round in yesterday's demanding cross-country test.
"He felt quite different from usual today," King said.
"The tension meant he went with his head high and flat in his body. It probably did not look pretty, but at least we managed to go clear.
"I just tried to blank everything out and pretend I was in a training session at Addington (in Buckinghamshire) where we had our training camp.
"I said to myself 'come on Mary, it's up to you' and it worked."
Cook's performance on Miners Frolic was a remarkable feat of composure and accuracy under the most suffocating pressure.
She called on all her previous top-level experience - two Olympic medals and the 2009 European individual title - to give Britain the round they required.
Cook added: "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, but now I am going for the individual."
World number one Fox-Pitt, who had a discount score of 53.30, despite jumping clear, has added another silver to his considerable medal collection.
"We are all being carried along on this wave of craziness, " said Fox-Pitt, of the London 2012 experience.
"This is something we have never experienced in our careers and never will again. It's just not normal."
Fox-Pitt was watched by his wife Alice Plunkett, the Channel 4 racing presenter, and their five-year-old son Thomas.
Plunkett said: "Gold was there for the taking. Obviously it is really disappointing that they didn't get it.
"It is not Zara's fault there were little mistakes. It's a team, it isn't about one person.
"William's time faults yesterday were expensive. There are a whole host of reasons - it's never down to one person."
However, there was disappointment when Scot David Florence crashed out of the canoe slalom today after finishing outside the cut-off for the final.
The 29-year-old from Aberdeen initially took sixth spot after going out eighth, but dropped down to tenth place and out of the competition.
The world number one and silver medallist from Beijing had been in line for a medal but his time of 106.16 seconds, which included two seconds, was not good enough.
Florence, nicknamed D-Flo, received huge cheers from the 12,000 capacity grandstand at Lee Valley white water centre as he powered his way down the 250 metre course.
He has another chance of a medal when he competes in the semi-final of the C2 - the two-man boat - on Thursday with team-mate Richard Hounslow.