Production of Spider-Man 4 is delayed, hence Paramount and Marvel Entertainment have pushed up the release of Thor by two weeks to 6th May 2011.
Thor was due to have opened 20th May 2011.
Disney have grabbed the 20th May slot for Pirates of the Caribbean: On Stranger Tides.
The shift came one day after Sony confirmed that lensing of Spider-Man 4 has been delayed because of script issues and that it may not make the 6th May deadline.
Sony advised it wasn't abandoning the date. The studio said it is still hoping to complete the film in time.
However, Paramount decided to act aggressively and not wait for the outcome, in case another studio acted first and put one of their films in the slot.
Paramount didn't comment on its decision, but it didn't come as a surprise to rival studios, as Thor is the first movie skedded for release after Spider-Man IV.
Two of the three Spider-Man pictures have opened the first weekend of summer; the third, over the Memorial Day holiday.
Should Spider-Man 4 stick to the 6th May release, insiders say that Thor would likely move.