An invitation to Prince Harry's fourth birthday party is among correspondence from Diana, Princess of Wales, being sold at auction.

The pink Spot the Dog invitation card, signed "lots of love Harry", was sent to a woman Diana had met during a visit to a hospital.

It is among four letters and cards written to the woman, who has not been named by auctioneers, going on sale at Keys in Aylsham, Norfolk on Friday.

One of the letters from Diana begins "Dear Doreen".

The invitation and Thank You card were originally sent to a woman who met the Princess when she was a patient in a hospital

Auctioneer Andrew Bullock

The collection also includes a Thank You card sent after the party at Kensington Palace, London, on 15 September 1988.

"An invitation to a prince's birthday party is something which doesn't come on the market very often," said auctioneer Andrew Bullock.

"I wouldn't imagine many would be in existence because I don't imagine many people would be invited.

Condolence message

"The collection is being sold by an autograph collector.

"But the invitation and Thank You card were originally sent to a woman who met the Princess when she was a patient in a hospital."

Diana's distinctive handwriting

A letter written by the Queen Mother shortly after the death of her husband, George VI, in 1952 is also up for sale.

That letter is expected to fetch about �1,000.

The letter was a reply to a message of condolence sent by BBC radio engineer Robert Wood.

He had helped the King to overcome a stammer when broadcasting.