Movie bosses behind Brad Pitt's upcoming zombie movie World War Z have opened up about the "insane" production problems which plagued the project, branding the shoot a "nightmare".

The horror film was initially due for release last year (12) but was bumped to this summer (13) amid rumours of issues with the script.

Executives behind the film have now revealed the full extent of the problems, confirming reports Lost writer Damon Lindelof was drafted in to help salvage the project.

The movie's ending was completely changed and led to weeks of re-shoots, while the $125 million (£78 million) budget ballooned to a massive $200 million (£125 million).

Marc Evans, president of production at the Paramount movie studio, has called the spiralling budget "literally insane" and a "nightmare," and admits costs were rising before they even realised the ending needed changing.

He tells Vanity Fair magazine, "It was, like, Wow. The ending of our movie doesn't work. I believed in that moment we needed to re-shoot the movie."

The film's star, Pitt, helped bring Lindelof on board to make the changes, and the writer explains, "(Pitt said) 'We started shooting the thing before we locked down how it was going to end up, and it didn't turn out the way we wanted it to'..."

However, he didn't think studio bosses would agree to his idea to re-shoot the ending, adding, "I didn't think anyone was going to say, 'Let's throw it out and try something else.' So when I gave them those two roads and they sounded more interested in Road B - which meant shooting an additional 30 to 40 minutes of the movie - I was like, 'To be honest with you, good luck selling that to Paramount.'"

The film, based on the 2006 post-apocalyptic horror novel by Max Brooks, is set for release in June (13).