A letter written by the Prince of Wales complaining over relentless questioning about life as a royal is to be auctioned.

The letter was sent in 1981 when he was on tour of New Zealand. In it he says: ''If one more child asks me what it's like to be a prince I shall go demented.''

The eight-page note, handwritten on Buckingham Palace paper, is one of 300 royal items to be sold at auction in the US later this year.

William doted on Harry as a baby, Diana's letter says

The items, including letters from Diana, Princess of Wales after her honeymoon and the birth of Prince Harry, give a rare insight into royal life.

Writing to a friend, Charles complains: "The real problem is keeping up my enthusiasm on each new day because I am beginning to get fed up with the amount of nonsensical rubbish I take all day and every day.

"If one more NZ child asks me what it's like to be a prince I shall go demented."

He finishes the letter: "Will you visit me when they strap me in a white apron and deposit me in some institution?"

Another letter by the Prince refers to Camilla Parker Bowles.

Penned in August 1980, less than a year before he married Diana, Charles refers to a vet tranquillising a horse.

He says: "My professional adviser (Camilla!) tells me that it was probably needed before being turned out in the field."

Collection

Other items in the auction include a note written by Diana after her honeymoon to a former member of staff from her family home who became a friend.

She says: "We had a wonderful honeymoon catching up on all that lack of sleep, and just being together made everything so perfect."

American businesswoman Alicia Carroll, who is selling the collection, said:

"Diana gave so many people gifts, letters and cards.

"If she had written you 50 letters, and you needed money for something, it would not be so bad to sell five or 10 of them.

"That's how I have been collecting them."

Mrs Parker Bowles mentioned in Charles' letter before his wedding

Ms Carroll, who runs a web site called everythingroyal.com, said she had 10,000 royal items and wanted to sell some of them.

But some items she has promised to never show.

"I have six letters that will never see the light of day, because I would not do anything to offend the Royal Family," she says.

"I will probably burn them one day."

One of Diana's letters expected to be in the auction was written after the birth of Prince Harry.

She tells a friend: "The reaction to his birth has totally overwhelmed us and we can hardly breathe for the mass of flowers that have arrived.

"William adores his little brother and spends the entire time pouring an endless supply of hugs and kisses on Harry, and we are hardly allowed near.

"I cannot quite believe I am now a mother-of-two."