Nine letters written by Princess Diana in the early years of her doomed marriage to Prince Charles are set to fetch up to £20,000 at auction.

The first letter is dated August 14, 1981, two days after Diana and Charles returned to Balmoral from their two-week honeymoon cruise aboard the Royal Yacht Britannia.

She told Maud Pendrey, her family’s former housekeeper, how her honeymoon was a ‘tremendous success’ and said she and Charles had a ‘glorious time’.

However, her words contradicted what she later told her voice coach in extracts which were screened in a Channel 4 documentary.

‘On our honeymoon, cufflinks arrive on his wrists,’ she said. ‘Two C’s entwined like the Chanel “C”. Got it. One knew exactly. So, I said: “Camilla gave you those, didn’t she?”

‘He said “Yes, so what’s wrong? They’re a present from a friend.” And boy, did we have a row. Jealousy, total jealousy. And it was such a good idea – the two “C’s – but it wasn’t that clever.”

Other letters to Ms Pendrey reveal Diana’s happiness about motherhood. In one, she described how she wanted ‘masses more’ babies after the birth of Prince William on June 21, 1982.

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Nine letters written by Princess Diana in the early years of her marriage to Prince Charles are set to fetch up to £20,000 at auction. Above: Charles and Diana in the grounds of Balmoral during their honeymoon, August 1981

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The first letter is dated August 14, 1981, two days after Diana and Charles returned to Balmoral from their two-week honeymoon cruise aboard the Royal Yacht Britannia. She told Maud Pendrey, her family’s former housekeeper, how her honeymoon was a ‘tremendous success’ and said she and Charles had a ‘glorious time’

She wrote on September 8 from Balmoral Castle: ‘Enclosed is a picture of an extremely proud and lucky mother and I just wondered if you’d like it!’

In her August 1981 letter, written from Althorp in Northamptonshire, Diana said: ‘I do hope you weren’t too tired after all the wedding activities and that you were able to see a lot from where your seats were,’ she wrote.

‘The honeymoon was a tremendous success, and we had a glorious time catching up on our lost energy and sleep.

‘I just wanted you both to know how deeply touched I am by your thoughtfulness and again a million thanks. Endless love and affection. Diana.’

Author Penny Junor also described in her book The Duchess that Diana’s differences with Charles emerged on their honeymoon.

Whilst the future King envisioned swimming, reading, painting, and writing thank-you letters, Diana had expected to chat.

‘He’d taken along his watercolours, some canvases and a pile of books by the Afrikaner mystic and writer Laurens van der Post, which he’d hoped he and Diana might share and then discuss in the evenings,’ Ms Junor wrote.

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Other letters to Ms Pendrey reveal Diana’s happiness about motherhood. In one, she described how she wanted ‘masses more’ babies after the birth of Prince William on June 21, 1982. She wrote on September 8 from Balmoral Castle: ‘Enclosed is a picture of an extremely proud and lucky mother and I just wondered if you’d like it!’

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On September 25, 1982, in a thank-you letter, Diana said: ‘We are so thrilled and delighted with your wonderful cardigans

‘Diana, however, was no great reader. She hated his wretched books and was offended that he might prefer to bury his head in one of them rather than sit and talk to her.

‘She resented him sitting for hours at his easel, too, and they had many blazing rows.

‘One day, when Charles was painting on the veranda deck of Britannia, he went off to look at something for half an hour. He came back to find she’d destroyed his painting and all his materials.’

In the September 8 letter, Diana added: ‘William has brought us so much happiness and contentment and consequently I can’t wait for masses more.

‘We are up in Scotland until the end of October which is particularly spoiling but greatly appreciated.’

And, on September 25, in a thank-you letter, she said: ‘We are so thrilled and delighted with your wonderful cardigans.

‘Thank you so much for spoiling William. He doesn’t deserve it! At the rate he is growing, it won’t be long before the cardigans fit!’

Two years later, when pregnant with Prince Harry, she still seemed content, later revealing that she and Charles were closest during her second pregnancy.

‘It was so kind of you both to have remembered us on our wedding anniversary,’ she wrote from Highgrove on July 29.

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Charles and Diana after arriving back at RAF Lossiemouth in Scotland following their honeymoon, September 1981

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On December 24, 1986, Diana wrote to Mrs Pendrey to express her condolences at the loss of her husband. She said: I wanted to write and say how very sad I was to hear about Mr Pendrey’

‘We were enormously touched by your card, and I just wanted to write and say a big thank you.

‘Unlike some married couples, we both remembered the occasion and luckily thought that the three years had flown past!

‘What with the baby arriving soon, our lives seem to be very busy, but we’d have it no other way!’

In another of the letters, Diana revealed her sense of humour and fondness for Ms Pendrey and her butler husband Ainslie.

She wrote: ‘When the helicopter took off from Althorp yesterday, we flew over your house, and I just wondered if you’d seen me waving…’

‘The family seems to be constantly expanding and Sarah’s baby is very sweet with extremely long fingers.

‘I just wanted you both to know that I must have seen one of the Pendreys – I was waving frantically!!’

The final two letters were written after Diana began her affair with James Hewitt, a cavalry officer in the British Army, who became her riding instructor, in 1986.

The first shows her compassionate side: ‘I wanted to write and say how very sad I was to hear about Mr Pendrey,’ she wrote on December 24, after the death of Ainslie.

‘I can only imagine that when something like this happens how empty you must feel.’

The second – written two years later – again showed her good manners.

‘It was so sweet of you to have sent us a lovely card for our anniversary, we were thrilled to bits to get it and very touched by your thoughtfulness,’ she said.

It is now 22 years since the letters first emerged in public.

They were sold for £22,000 by Keys auction house, in Aylsham, Norfolk, to hotelier Michael Rockall, who displayed them at the Whittlebury Hall, in Towcester, Northamptonshire.

‘I think this collection is amazingly exciting and I am overjoyed to have bought it,’ he said at the time.

‘The price was pretty near the level we wanted to go to, and I thought it might go abroad.

‘I feel very strongly that something like this should stay in England.

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Charles and Diana stand on the banks of the River Dee in the grounds of Balmoral during their honeymoon, August 1981

‘I view it as being a particularly personal collection that the Princess wrote to someone who she would have known personally for a long time.’

When the hotel was merged with the local golf course, the collection, which also includes 14 Christmas cards, a signed photograph and a Royal wedding invitation, became the property of new owners Jeff and Carol Sargeant.

They are being sold via California-based firm Julien’s Auctions later today with an upper estimate of $28,000 (£23,000).

The auction house said on their website: ‘The collection of letters, encompassing thank-you notes, holiday greetings, and other correspondence, provides a rare look at Diana’s life beyond the carefully crafted public image.

‘In her own elegant script, Diana’s warmth and genuine connection with others shine through.

‘These letters reveal a woman who, despite her royal status, remained grounded and appreciative of the people in her life, from her staff to her close confidants.’

Diana’s marriage to Charles ended in divorce in August 1996, a year before she was killed in a car crash in Paris.

The King went on to marry Queen Camilla in 2005.