Powell lands Burghley title after clear showjumping round sees off her rivals
Powell, partnered by the 17-year-old Lenamore, became the first woman from New Zealand to win the event in what was her first four-star victory.
The 37-year-old, who is based near Kelso in the Scottish Borders, finished with 38.7 penalties, 4.2 above second-placed William Fox-Pitt on Seacookie.
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Hide AdPreviously she had amassed five top-10 finishes at Badminton and Burghley with Lenamore and this was undoubtedly her greatest result, becoming the fourth New Zealander after Mark Todd, Andrew Nicolson and Blyth Tait to win Burghley.
"That was great. The horse has been such a star all week. He just jumped - he loves it," said Powell.
"I'm just so pleased with the way he jumped. This is just amazing – it just doesn't happen to little girls like me. I'm absolutely chuffed."
Topping the leaderboard after the cross country, Powell entered the showjumping with a fence in hand but sailed over the course without mishap.
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Hide AdHer position was strengthened when her two closest rivals – Fox-Pitt and Huddersfield-born Oliver Townend – both incurred penalties.
Townend and Carousel Quest, attempting to become the first combination to win Burghley for two years running, started the day just 0.9 penalties behind Powell.
But they fell foul of two fences and dropped from second to fourth place as a result.
Fox-Pitt drew one time penalty and moved up to second place as his bid to win Burghley for a record sixth time came to an end.
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Hide AdAustralia's Clayton Fredericks and Be My Guest II produced a clear round on the show jumping to improve their overnight position from sixth to third.
A single downed fence lifted Mary King and Apache Sauce up a single place to fifth while Fox-Pitt finished sixth on his second ride Macchiato.
Powell's success proved an early wedding present for Lenamore's owner Lexi Mackinnon, who gets married next weekend.
Powell added: "When we arrived here on Wednesday I said to Lexi that I hadn't got her a wedding present yet, and she said, 'Oh, you can win Burghley,' so now I don't have to get her a wedding present."
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Hide AdFox-Pitt, meanwhile, paid tribute to Seacookie's performance.
It was a result that left him heading the race to win this year's HSBC Classics Series and its 92,000 top prize. The series, won by Fox-Pitt in 2008, concludes in Pau in November, which is the fifth of this year's four-star events, and, rewards the rider with best aggregate results from those competitions.
"Seacookie is a brilliant horse," he said.