As a daughter of the noble Spencer house, Princess Diana wore the diamond tiara on her wedding day in July 1981.
She eventually handed it back to her brother Charles, the current Earl Spencer, and the present custodian is his third wife Karen.
If Harry, 33, asks his uncle for the tiara, he could loan it to his fiancee for the big day at St George’s Chapel, Windsor Castle, on May 19.
If the Queen wants to show she is sure of this marriage, she will offer Meghan, 36, the use of a minor royal tiara, as she did with Kate Middleton, who chose the 1936 Cartier halo headpiece which had belonged to the Queen Mother and was given to the Queen for her 18th birthday.
The Duchess of Cambridge’s parents commissioned oak leaf and acorn diamond earrings, designed to complement the tiara’s delicate design.
Kate has also worn the Lotus Flower tiara, once owned by the Queen Mother, and the Cambridge Lover’s Knot tiara, a wedding present to Diana from the Queen and taken back into the Royal Collection after the princess’s death.
It is thought unlikely the Queen will lend Meghan any larger, more important tiaras that are associated with her role as head of state, which include the George VI Sapphire.