The money, to be given to the Princess Diana Memorial Fund on Monday, comes from stamps, presentation packs and first-day covers - stamped envelopes - sold to collectors.

These so-called philatelic profits do not include profits from the sale of the Diana stamps for postage.

But leading stamps expert Peter Jennings said: "The Post Office announced on several occasions in the media that they intended to give all profits from the sale of the Princess Diana stamps to the Memorial Fund.

"It is deplorable that on 2 February 1998, the day before the stamps were issued to the public, the Post Office announced that only philatelic profits would be going to the Memorial Fund."

Princess Diana: The memorial fund will receive £9m

But the Royal Mail said an initial press release saying all profits would be donated was inaccurate and that it had always been the intention to give only philatelic profits.

The stamps, all 26p first class, featured formal and informal poses. Each stamp had a purple border as a mark of respect.

The Queen gave her approval to the designs soon after Diana's death, but the release was delayed.

It was understood members of the Spencer family were concerned that Prince William and Prince Harry would be upset at seeing their mother's face on stamps soon after her death.

On the day of release, thousands of people queued for them at post offices.