Princess Diana's letter also signed by Harry and William up for sale for £18,000

To have an item signed by Diana, William and Harry is quite special," a spokesperson said.

A letter from Princess Diana with the scrawled signatures of young Princes William and Harry has gone up for sale £18,000.

The Princess had written the note to thank a police officer for putting on a motorbike display for William's birthday party. The boys, then aged seven and four, added their names to the bottom.

According to a report in Daily Mail, sergeant George Plumb of the Metropolitan Police and the Special Escort Group had put on the dazzling motorbike display for William, Harry, and their friends on the grounds of Kensington Palace in 1989.

Diana's letter, dated June 21, 1989 and written on Kensington Palace headed paper read:

"Dear Sgt Plumb, It was so very kind of you and your team to have come here today on William's birthday - I cannot begin to tell you what pleasure the display gave to all those little people and their mothers!

"I know how particularly busy you all are at the moment, so it meant even more to us that you and the motorbike boys were able to take part in the birthday celebrations!

"This comes with our warmest possible thanks. Yours sincerely Diana, William, Harry."

By being "busy", Diana refers to the public transport workers' strikes taking place in London at the time, affecting rail, underground, and bus services.

A spokesperson told the Daily Mail, "She (Diana) was particularly grateful to the team for being able to fit this on a day already full of official engagements, which was not helped by the rail, underground, and bus strikes - something which strikes a chord with today... To have an item signed by Diana, William and Harry is quite special."

The letter left sergeant Plumb's estate six years ago when they were bought by a private collector. It will be auctioned on January 10.