Former Grand National-winning jockey Graham Lee suffered spinal cord damage in a fall and remains in intensive care, says the Injured Jockeys' Fund.
The 47-year-old was injured in a fall at Newcastle Racecourse on Friday.
Lee was unseated from his mount Ben Macdui as the stalls opened and taken to Royal Victoria Infirmary Hospital in Newcastle.
The IJF added it was too early "to predict the extent of long-term recovery" for the Irishman.
In an update on Galway-born Lee, released on Sunday, the body said an MRI scan "has shown that he has suffered an unstable cervical fracture causing damage to the spinal cord, as well as damage to blood vessels in the mid-cervical region.
"He remains in ITU with respiratory support at the Royal Victoria Infirmary Hospital in Newcastle, but it has been possible to reduce his sedation.
"This is a very serious injury and, at this early time, it is not possible to predict the extent of long-term recovery.
"Graham's family are very grateful for the many messages of support that they have had."
Lee rode more than 1,000 winners over the jumps, including Amberleigh House for trainer Ginger McCain in the 2004 Grand National.
He switched to riding on the Flat in 2012 and won the Gold Cup at Ascot three years later with Trip To Paris.