A touching wartime letter written by Elizabeth II as a young princess to a Canadian soldier is going on sale.

The letter, penned from Windsor Castle on August 26, 1940 to Lady Aston, includes a signed photograph of the castle.

The future queen had promised to return the picture to a Canadian soldier in convalescence at Cliveden.

It reads: "I am so sorry not to have sent the photograph before but here it is at last. I do hope the soldier has not gone away. 

''It was entirely my fault that the photograph was not returned at once and I do hope he will forgive me.''

The small black and white photograph showing Buckingham Palace at night is signed to the back for both Princess Elizabeth and Princess Margaret.

It is annotated by Elizabeth ‘Buckingham Place floodlit, May 1935, given to me by Bobby in London, 1940’.

The vendor's father was the recipient of the picture and the letter. 

A member of the Canadian Army, 1st Division, Signal Corps, he had met both Princess Elizabeth and Princess Margaret when hospitalised on the Cliveden estate. 

The vendor recalls: “My father had some pictures on his bed which included the picture of Buckingham Palace. 

''Princess Elizabeth asked if she could borrow the picture to show her father the King and later returned it with a letter of apology for the delay.".

Offered together with letters from Mary Astor, it is expected to fetch £2,000 when it from May 31 June 16 at Chiswick Auctions.