The script of 118 pages, marked "Private and Confidential", was spotted under a table at a pub. The script revealed how film bosses have made several dramatic changes from JK Rowling's book 'Harry Potter And The Deathly Hallows', reported Sun online.
"I couldn't believe my eyes. I know guys that work on the film drink in the pub, but it's very embarrassing that someone could just leave it there. I'm glad I've helped to stop any surprises being leaked," said the reader who found the script.
Studio Warner Bros, who are producing the film, launched an investigation into how the inch-thick booklet was lost as sources told how the film's crew went on a major drinking session.
"They'd got behind schedule in filming but then caught up, so they went out for a celebratory drink which led to many more. They usually come down for a session on a Friday, but because it was Easter they were in a day early. Daniel Radcliffe is the only member of the cast who has been known to come down and have a laugh with them, but he wasn't there that night," said a source.
The script shows how the final Potter story has been split into two parts to keep fans in suspense. The first film - with Daniel's co-stars Emma Watson and Rupert Grint back in their usual roles - is due for release in November. Part two will follow in July next year.