A reader found 118 pages detailing changes being made to "Harry Potter and The Deathly Hallows" at a bar in Kings Langley, Hertfordshire, and turned it over The Sun, the tabloid reported.
The script was marked "private and confidential," The Sun said.
Warner Bros. thanked the Sun for returning the inch-thick script, then launched an investigation into who lost it, The Sun said, citing sources who reportedly saw the film crew during "a major drinking session" Thursday night.
Filming is nearly complete on part one of the two-part "Harry Potter" finale. The first is due out in November with the final installment scheduled for summer 2011 release.
"We are grateful to The Sun for the safe return of this script," a Warner Bros. spokesman said.