Prince William is to be made a Royal Knight of the Garter – the most senior British order of chivalry.
He will be officially appointed by the Queen at a service at St George’s Chapel at Windsor Castle later.
The Order of the Garter – established by Edward III in 1348 – honours those who have contributed to national life or served the Queen.
William will become a Royal Knight Companion, which recognises his seniority within the Royal Family.
There are 26 full members of the Order of the Garter, including the Queen and the Prince of Wales. The Queen chooses the other 24 members, known as Knights and Ladies Companions, without the advice of ministers.
However, other members of the royal family and foreign monarchs are also made additional members of the order – known as either Royal Knights and Ladies, or Stranger Knights and Ladies.
William’s father, Prince Charles, was appointed in 1958, the Princess Royal in 1994, and the Duke of York and the Earl of Wessex in 2006.
The Duke of Edinburgh was made a Royal Knight in 1947 – the same year he married Princess Elizabeth.
The numbers of the 24 Knights Companion were depleted recently with the deaths of the conqueror of Mount Everest, Sir Edmund Hillary, and former Prime Minister Sir Edward Heath.
Their places have been taken by Lord Luce, Lord Chamberlain from 2000 to 2006, and Sir Thomas Dunne, who has been Lord-Lieutenant of Hereford since 1977 and chairman of the Lord Lieutenants Association.
Former prime ministers are usually made members of the order soon after their retirement from office.
The order already includes former Prime Ministers Baroness Thatcher and Sir John Major.
New members of the order are traditionally announced on St George’s Day but the ceremonies take place in June, on the Monday of Royal Ascot week, known as Garter Day.
Prince William, who will be the 1,000th Knight in the Register, will wear a blue velvet cape and black velvet hat with white ostrich plumes for a procession from the castle to the chapel.