In the letter, the late princess thanks a police sergeant for participating in a motorcycle parade at William’s birthday party. The boys, then aged seven and four, added their signatures at the bottom of the letter.
Sergeant George Plumb of the Metropolitan Police and his special group gave a dazzling motorbike show to Princes William and Harry and their friends on the grounds of Kensington Palace in 1989.
The late princess sent the letter on June 21, 1989, written on paper at Kensington Palace.
The letter read: “Dear Sergeant Plumb, it has been very kind of you and your team to come here today on William’s birthday – I can only tell you what a pleasure it was for all these little children and their mothers!”
She added, “I know how many heavy tasks you have at the moment, so having you and the motorcycle team participate in the birthday party meant a lot to us.”
The comment about “Plumb and the motorcyclist getting busy” refers to strikes by public transport workers in London at the time, which affected rail, tube and bus services.
A Sotheby’s spokesperson said: “This letter is personally signed by Diana, Princess of Wales, HRH Prince William and HRH Prince Harry, dated 21 June 1989.”
Sergeant Plumb sold the letter six years ago to a private collector, who is now putting it up for sale again at auction on January 10.