The Countess of Wessex will set off on a 450-mile palace-to-palace cycling challenge from Edinburgh today.
Sophie and her team will pedal from the Palace of Holyroodhouse to Buckingham Palace in support of the Duke of Edinburgh's Award to celebrate the scheme's 60th anniversary.
She will be waved off on the Diamond Challenge by the Duke of Edinburgh and her husband Prince Edward, the Earl of Wessex.
Edward will wave a flag to start the week-long cycle.
He and Prince Philip will meet young people who have completed their own Diamond Challenges in the award scheme's 60th year.
Sophie is joined by six team members, including riders drawn from the four Royal regiments she is linked to - 5th Battalion The Rifles, RAF Wittering, Queen Alexandra's Royal Army Nursing Corps and Corps of Army Music.
The team also includes members from the Army Physical Training Corps and Boardman bikes.
The countess is also the ship's sponsor of HMS Daring, which will be at sea during the ride but cyclists on board plan to show their support by completing the distance on the vessel's static bikes.
Speaking during a recent training session, Sophie said: "I think if you'd told me a few months ago that I would be able to cycle 60 miles in one go I would have laughed, but here I am doing 60 miles.
"It's been hard and there have definitely been moments when I have wondered what on earth I have decided to do!"