ssrs 2008 report timeout

Hello,
The ssrs report is calling a stored procedure to populate data...
The stored proc. takes about 15 minutes to complete.
Therefore the report gives a timeout message after 10 minutes.
For business reasons, the stored proc. should remain as is.
Question:
How can I increase the timeout so that this timeout is not shown and the report is populated even if it takes longer than 10 minutes to complete?
Thanks