The Queen made sporting history when she became the first reigning monarch to win Royal Ascot's Gold Cup - and described the experience as a very big thrill.
With a clap of her hands and a beaming smile, she celebrated the moment her thoroughbred filly Estimate finished first, as her family jumped for joy behind her in the royal box.
Princesses Beatrice and Eugenie had joined their grandmother for a day at the races and could not contain their excitement during the dramatic finish.
Queen Elizabeth II with her racing manager John Warren after her horse, Estimate, won the Gold Cup on day three of the Royal Ascot meeting
The Queen's smile lasted long after her horse crossed the finish line and she was still beaming when she joined jockey Ryan Moore and trainer Sir Michael Stoute in the winners' enclosure for the trophy presentation.
Today's victory is the Queen's biggest win since her horse Dunfermline ridden by jockey Willie Carson won the classic races - the Oaks and St Leger - in her Silver Jubilee year 1977.
Sir Michael said the Queen greatly appreciated the victory: "It's a special thrill to win this race for the Queen and it will have given her enormous pleasure - she really loves this game and it's a great recreation for her.
"She told me it was a very big thrill and thanked everybody involved."
The Queen has been a passionate horse owner and breeder for much of her 61-year reign and even whiled away her 87th birthday in April, at the yard of a west country trainer, casting her eye over thoroughbreds.
Her horses have also won four out of the five flat racing classics - the 1,000 Guineas and 2,000 Guineas, the Oaks and the St Leger - with only the Derby eluding her.
Estimate's victory was the first time a reigning monarch has won the Gold Cup in its 207-year history.
Princess Beatrice (left) and Princess Eugenie cheer as Britain's Queen Elizabeth II horse, Estimate, wins the Gold Cup
The Queen was hoping her horse would surpass its achievements of last year when she won the Queen's Vase at Royal Ascot.
But Estimate lived up to her billing as pre-start favourite and, as the race over 4,000 metres drew to a close, she made her way from the middle of the pack, took on the front runners and beat them to the line.
The victory is an added boost for the Queen, who has been attending the famous annual meet this week without the Duke of Edinburgh, who is convalescing at Windsor Castle after exploratory abdominal surgery.
Still wearing the Queen's racing silks of purple and scarlet with gold braiding, jockey Moore said of Estimate, the 7-2 favourite: "She won well, and to do it for the Queen is fantastic.
"To win the Gold Cup in her colours is exceptional."
The Queen was scheduled to hand out the winning trophies for the Gold Cup but the Duke of York, part of the royal party, stepped in to do the duties.
Prime Minister David Cameron also praised the Queen's win in a tweet:
William Hill said it was left reeling after the win as it had paid out £1.5 million to punters who backed Estimate.
The bookmaker said odds for the Queen's horse were 7-1 this morning before shortening to 5-1, while at Royal Ascot it was also the favourite with odds of 7-2.
A spokeswoman said: "This was a true patriotic punt on the Queen's runner Estimate, and her victory has handed us a stinging £1.5 million payout.
"Her Majesty is flying the flag for British bettors today."
Despite scooping £155,960 in prize money as the owner, the Queen will not have made anything from a small wager as she does not bet.