The unnamed 25 year old suffered horrific injuries after a flying sequence using a harness went wrong, and the blast - which was part of the stunt - caused him to plummet to the ground.
He was conscious after the fall, but told colleagues: "I can't feel my legs."
Paramedics fear the stuntman might have been paralysed and he was rushed to hospital on Wednesday for emergency treatment.
A source said: "The guy was rigged up to a harness for the scene and was flying through the air. It is thought he may have been caught by the explosion and hit the ground very hard. Everyone is just hoping he makes a good recovery."
Pre-production work is underway in preparation for 'Harry Potter and the Deathly Hallows' - the seventh movie about the young wizard - and prospective stunts were being tested at the Leavesden studios near Watford when the accident happened.
A spokesman for the studio said: "We can confirm a member of the 'Harry Potter' production was injured in an accident. We are awaiting further news. Out of respect for the family we are unable to comment further."
©2009 AOL Australia Pty Limited in association with Bang Showbiz. All rights reserved. This material may not be published, broadcast, rewritten or redistributed.
Related Article: Daniel Radcliffe says new Harry Potter is like 'Trainspotting'
