Pippa Funnell beat Piggy French by just 0.1 penalties to win the Burghley Horse Trials for the first time in 16 years.
The 50-year-old, riding MGH Grafton Street, just edged out French, 39, who was looking for a double after her Badminton victory in May.
Funnell led from day one and won the event for the first time since she completed the Grand Slam in 2003.
Former winner Oliver Townend completed a British top three on Ballaghmor Class.
British rider Sarah Bullimore finished fourth on Reve du Rouet, while Imogen Murray and Gemma Tattersall finished in the top 10.
Last year's winner, New Zealander Tim Price, finished fifth after riding clear in the show jumping on the final day.