Zara Phillips will be cheered on by the Duke and Duchess of Cambridge as she tries to earn an Olympic place at the Badminton Horse Trials.
William and Kate will be in the stands to watch Ms Phillips, the Queen's granddaughter and one of Britain's leading equestrians, compete against a top international field next month.
The royal couple will attend the final day of the five-day event, which attracts thousands of horse enthusiasts every year and is a major highlight in the equestrian calendar.
The Duke has received a number of invitations to Badminton but this is the first time he has been able to attend.
The royals will watch the showjumping on May 7, the last element of the trials held in Gloucestershire, which include a dressage element and a gruelling cross-country course.
Competitors will receive their prizes from William and Kate, who are expected to be rooting for the Duke's cousin Ms Phillips, a former equestrian world champion.
She guided her horse High Kingdom to a 10th-place finish at the Burghley horse trials last September, and a strong Badminton display would keep her in the thoughts of selectors for London 2012.
William and his brother Prince Harry are both keen riders who regularly play polo but Kate is thought to be allergic to horses.
The royal couple will celebrate their first wedding anniversary on Sunday.