If it had been written by anyone else, or about any other dog, a hastily scribbled note in pencil answering a vet's questions about a sick pet would have been discarded years ago.

But this is a missive from the Queen and, together with a more heartfelt letter penned a few days later, concerns her beloved corgi Susan, the matriarch who founded perhaps the most famous canine dynasty in the world,

Now the correspondence, and the story behind it, will be heard by millions of viewers when Dereham resident Carole Harrisson features on a special edition of the Antiques Roadshow filmed at Kensington Palace.

Her father Harold Swann was practising at the Veterinary Centre and Hospital in London Road, King's Lynn, in January 1959 when he received a visitor perplexed by a dog's illness.

Mrs Harrisson said: 'A footman came into the surgery and said could my father look at the dog. He looked and said he would have to know a few more details about the dog, so on a scruffy note pad he put down a list of questions, and the footman gave it to the Queen.'

The note was duly returned with the answers written in the royal hand and signed 'ER'. Asked how long Susan had been getting bigger, the Queen responded: 'Possibly became noticeable a week ago. No idea – she's always been fat!'

Susan was no ordinary corgi, but a special present to the-then Princess Elizabeth from her parents on her 18th birthday. She went on to accompany the princess on her honeymoon, and her bloodline is continued in all of today's royal corgis.

After her death on January 26 1959, she was buried in the grounds of Sandringham with a memorial designed by the Queen.

Three days later, the Queen penned a more formal letter to Mr Swann, this time written in pen on Sandringham-headed paper and delivered in a hand-addressed envelope.

The Queen wrote: 'I would like to thank you for all you did for my dear old Susan when she became ill, and for the immense amount of trouble you took in getting her sent to Cambridge and for all the care she had while she was there.

'I had always dreaded losing her as I had had her since she was six weeks old but I am so thankful that her suffering was so mercifully short – she was very happily beating for us out shooting on the Friday before!'

Antiques Roadshow director Louise Hibbins said she had been hoping to find glimpses of the Queen that the public do not usually see, something the letters to Mr Swann provided.

She said: 'I think these little insights show a sense of humour and a total love of her dogs, and maybe with the life the Queen has had to live you get more from your animals.'

Mrs Harrisson said she knew her father had a letter from the Queen, but he did not reveal much about his service for the royal family in the 35 years before his death in 1994.

She said: 'It makes me very proud of my father to be involved and do a lot of Sandringham work. Being a professional, he did not say a lot.'

She was one of 600 people to contact the programme when it announced its Diamond Jubilee special, and was of one of only 20 to be invited to the filming on May 7.

Although the show's experts placed a value on her items, Mrs Harrisson has no intention of parting with them: 'It's a personal letter from the family and I would not sell it for a million pounds.'

To see how much the letters are worth, watch the Antiques Roadshow at 7.30pm on BBC1 on Sunday June 10.