PRINCESS Diana’s last official letter – written to a humanitarian campaigner who was hijacked by armed paramilitaries – is to go under the hammer.

Diana sent the hand-signed note on Kensington Palace paper to Dilys Cheetham, of Richmond, North Yorkshire, on August 11, 1997, less than three weeks before she died in a car crash in Paris.

The princess, who had recently returned from a three-day visit to Bosnia as part of her crusade against landmines, thanked Ms Cheetham for delivering aid to refugee camps.

Diana wrote: “There was not enough time for me to visit the Mostar area while I was in Bosnia, but I was able to visit a number of anti-personnel landmine victims and their families.

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“I could not help but be deeply moved by the experience.”

She added the trip “hardened my resolve to ensure the world does not forget that those who have been so needlessly maimed by these terrible weapons will need care and support for many years to come”.

Ms Cheetham, who founded the charity Nobody's Children, had also distributed medical supplies and toiletries for children arriving at hospital in Mostar, which had been collected by children at Romanby Primary School, Northallerton.

In a subsequent aid mission, Ms Cheetham was stopped by armed Serbians who demanded her aid-filled lorry as she travelled alone through the Republic of Serbska.

Speaking in 1999, Ms Cheetham told the Northern Echo: "They wanted my vehicle - and I don't believe in dead heroes. They pushed me around a bit, but I managed to get them talking about football which is a great leveler.

"The danger does not bother me. I could get run over crossing the road tomorrow."

Before her death aged 65 in 2006, Ms Cheetham sold the letter in a charity auction to raise money for landmine victims and the note has recently emerged among other papers at a pawnbroker.

Mark Huddleston, head of antiques at Fellows Auctions, Birmingham, said: “This is an incredibly important letter in terms of modern history, more so than the Edward VIII love letter we sold for £5,000.

“The letter is moving not only because of its heartfelt message but also because it would have been the last ever written by her.”

The letter is expected to fetch £3,000 at the sale on Monday.