An unpublished letter written by Charlotte Bronte has amazed experts by fetching more than ten times its estimated value at auction.

The letter to one of her closest friends, which discloses intimate views on passion and marriage, sold for almost £70,000 when it went under the hammer in London.

Phillips auctioneers estimated it would sell for between £4,000 and £6,000 but bidding went through the roof during the sale of books and manuscripts on Friday.

It finally stopped at £69,700 when the three pages of text, complete with an envelope bearing a Bradford penny postmark and the date May 16, 1840, went to an anonymous telephone bidder.

Mike Hill, director of Bronte Parsonage Museum, said the Bronte Society had bid for the letter. "We were very disappointed not to get it for our collection. We would have loved it because it needs to be in the public domain," he said.

"Bronte material is more highly valued than almost any other writer's but this price was very high even by Bronte standards."

He said the letter had not been seen since the turn of the century but during the three days it had been on view at the auctioneers, Bronte Society member Margaret Smith, who is editing Charlotte's letters, had looked it over.

Charlotte wrote the letter from her father's rectory in Haworth to Ellen Nussey whose clergyman brother Henry unsuccessfully proposed to her.

In it, the 24-year-old warns her friend to resist being ruled by her passions but advises selecting a husband on the basis of respect which, she assures her, will turn into "moderate love at least."

Charlotte goes on to predict an early marriage for Ellen but, incorrectly, a life of spinsterhood for herself, despite an admission that she is easily wooed.

"If I made you my father confessor, I could reveal weaknesses in my character which you do not dream of - I do not mean to intimate that I attach a high value to empty compliments, but a word of panegyric has often made me feel a sense of confused pleasure which it required me strongest efforts to conceal," she writes.

Elsewhere, the letter contains unflattering references to other mutual friends, whose names were scratched out, possibly before the letter was entrusted by Ellen to author Elisabeth Gaskell while she was researching her biography of Charlotte.

A spokesman for Phillips said the huge interest aroused by the letter had been totally unexpected.

Converted for the new archive on 30 June 2000. Some images and formatting may have been lost in the conversion.