Holmes was practicing a stunt on a harness when a planned explosion caused him to fall and fall to the ground.
The former gymnast has been Ratcliffe's stunt double in all of the Harry Potter movies.
Holmes was first taken to Watford General Hospital but has since been transferred to a spinal injury hospital.
The Daily Mail
A spokesman for Leavesden studios said: ‘We can confirm that a
member of the Harry Potter production was injured in an accident that took place on Wednesday.
‘Out of respect for his family, we are unable to comment further at this stage, but can confirm that it was not a member of the cast.’
Radcliffe is currently in New York. It is not clear if he has been informed of Holmes' injury.