A rare Harry Potter book bought for £1 (NZ$1.85) at a table top sale has fetched £28,500 (NZ$52,790) at auction.

The 1997 edition of the first in JK Rowling's seven-part series was discovered by auctioneer Jim Spencer in a banana box full of books.

The hardback, of which only 500 copies were published by Bloomsbury, was sold to an anonymous bidder at Bishton Hall in Staffordshire on Tuesday — Miss Rowling's 54th birthday.

Around 300 of these first editions went to libraries at the time. Auctioneer Charles Hanson said: "It was a magical moment... and [it] is an investment like no other."

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Mr Spencer said he was in "disbelief" when he found the copy — which he described as the "Holy Grail for book collectors".

He said: "It is something that is always on your radar, and you are thinking, 'will I ever turn one up?' I was pretty shocked.

"I must get two or three calls a week saying, 'I have got a Harry Potter first edition', but you can generally put most to bed and rule them out within half a minute."

Mr Spencer, who spotted the copy after he was called to a client's house in Staffordshire to value three boxes of books, said experts looked for "issue points" to determine if it was genuine. He noticed a printing error in a list of Harry's school "supplies". The repetition of "1 wand" in the list was corrected in later editions — confirming this was an original. The the word "philosopher" was also initially spelt wrongly on the back.

The book's owner, 54, who asked not to be named, said: "I bought it along with three or four others to read on holiday about 20 years ago. I thought nothing of it at the time."