A new coat of arms designed to mark Prince William's coming of age contain a tribute to his late mother Princess Diana - in the shape of a small red escallop shell.

The shell, which derives from the Spencer coat of arms, was used by Diana and has been incorporated in the design at the young prince's insistence.

The arms chosen for the 18-year-old prince also draws on the royal arms used by the Queen and his father, Prince Charles.

A royal licence is being drafted to grant the coat of arms officially to the prince, who celebrated his 18th birthday last month.

The escallop motif has been borne by the Earls Spencer since the 16th century and was a popular symbol for medieval pilgrims.

It appears four times on William's arms - in the centre of the three-pointed label which adorns the shield; on the necks of the lion and unicorn, which support the shield; and on the lion device above the shield.

Prince William, as heir apparent to the heir apparent, his father, will be the only one of the Queen's grandchildren to be given a three-pointed label on his arms.

Three-pointed labels identify the children of a monarch while five pointed labels are normally used for grandchildren.

Peter Gwynne-Jones, responsible for royal heraldry, said: "It is a welcome innovation to incorporate maternal symbols into the Royal Family's arms and it is something that Prince William and his family wanted to do.

"In the fullness of time, Prince William's arms will change, as the Prince of Wales's shall, but a precedent has been set here that others in the Royal Family may well follow."

The design has been approved by both Prince William, his father and the Queen.