The Queen's granddaughter Zara Phillips and England rugby star Mike Tindall are set to marry.
The couple will exchange vows in an Edinburgh church surrounded by their families and friends.
But their nuptials are being billed as a low-key affair - in stark contrast to the recent wedding of the Duke and Duchess of Cambridge.
William and Kate's televised marriage was watched by a worldwide audience of hundreds of millions and attended by a host of celebrities and dignitaries.
The doors of the Scottish capital's Canongate Kirk, where Tindall, 32, will first catch sight of his 30-year-old bride in her wedding gown, are to be closed to the press and public when the ceremony begins at 3pm.
But large crowds are still expected in Edinburgh's Old Town to watch the arrival of the Royal Family who are all expected to attend.
The Queen and Duke of Edinburgh will join Zara's parents the Princess Royal and Captain Mark Phillips at the kirk. Also believed to be attending are the Prince of Wales and Duchess of Cornwall, William and Kate, Prince Harry, the Duke of York and his daughters Princess Beatrice and Princess Eugenie.
Tindall is likely to have invited some of England's best-known rugby players who are colleagues and friends.
Like most modern brides Zara is expected to say she will "honour" her groom rather than "obey" when she exchanges marriage vows with him during the ceremony conducted by the Reverend Neil Gardner.
The couple appeared relaxed on Friday when they arrived for a wedding rehearsal at the kirk, and waved to acknowledge the crowds who cheered them.