To the untrained eye, it might have looked like just another unwanted book left at a charity shop which would sell for small change.

But when a scruffy looking copy of Graham Greene's Rumour at Nightfall appeared in a box of donated books at Oxfam's bookshop in St Giles, Andrew Chapman - one of the volunteers there - thought it might be worth a second look.

Fortunately for Oxfam, Mr Chapman's shrewd eye and expertise paid off and the book went on to fetch £15,000 at auction - more than twice its listed price.

In doing so, it has become the most expensive book Oxfam has ever sold - a feat Mr Chapman, in typically understated fashion - said was a "very nice present" for the charity.

The proceeds will be used to fund a variety of Oxfam projects across the globe.

The airline pilot, who works as a volunteer rare books specialist at Oxfam in his spare time, said he was "astounded" by the sale price.

He said: "I was sitting in the auction house and was amazed to hear the price going up and up and up.

"It was incredibly exciting and quite emotional. I was totally amazed."

The book - Graham Greene's third work and one of only 1,200 first edition copies published in 1931 - had been listed for sale with a guide price of £6,000 to £8,000.

Mr Chapman said it had arrived on the shop's doorstep about five months ago in a box of about 10 books, none of which at first glance appeared to be particularly special.

He said: "We think the box had been brought in after someone had died.

"We're not quite sure who dropped it off, but at first glance none of the books really stood out.

"I later realised this book would be of some value and started making the necessary arrangements for it to go to auction, but never did we think it would fetch the price tag it did."

Mr Chapman said the bookshop in St Giles, which was the first of Oxfam's 100 bookshops, often sells books for between £500 to £2,000 at auction.

He said: "We do come across a number of rare books, but have never had anything like this before.

"It's a real first and at £15,000 is a very nice present."

Mr Chapman said: "The important thing to remember here is the sale of this book will really make a difference to one or more Oxfam projects."

The book was sold at Bloomsbury Auctions in London.

It was bought by Peter Harrington Books, one of London's top rare book dealers.

Graham Greene, who is better known for novels such as The End of the Affair and Brighton Rock, was believed to dislike Rumour at Nightfall, which he described as "of a badness beyond the power of criticism properly to evoke".