A rare first edition of Harry Potter and the Philosopher's Stone, set to be thrown away, is expected to sell for up to £40,000.

Errors helped an auctioneer identify the hardback as one of 500 copies from the book's first print run.

Daniel Pearce, who runs NLB Auctions in Paignton, Devon, found it among the possessions of a deceased man from Brixham.

He said: "It's our biggest find. The family are delighted, they didn't expect it at all... it will be important money for them, and it has been sat on a shelf for 20 years."

"It's something everybody dreams of really," said Mr Pearce.

"You pop into a property hoping to find something valuable."

He said he found the copy on top of a pile of items earmarked for the bin.

Mr Pearce said from the first print run 200 were sent to bookshops and 300 were distributed to public libraries.

He said: "This is one of the 300. As it is a library edition, it is in cracking condition considering.

"Usually these come to the market beaten up and spines damaged. This one is great."

Mr Pearce said on the first page of the book there was a £2 pencil mark, which was the cost the library sold it for when it was checked out.

He said the misspelling of philosopher's on the back of the copy was among the things that identified it as a first edition.

"It is missing the 'o' at the end," he said.

Other differences found in first editions include "inside the first couple of pages you will notice the title line has the numbers extending from 10 down to one".

Mr Pearce said another copy, in a similar condition, sold in November for £42,000 including the fees.

"There are still a few out there," he added.

"I believe they are the most expensive contemporary first edition you can get now."