Champion jockey Jim Crowley and fellow rider Freddy Tylicki were taken to hospital on Monday after both were involved in a four-horse fall at Kempton.
During the third race of the flat meeting, Tylicki's Nellie Dean went down heavily after seemingly clipping the heels of Madame Butterfly.
Crowley, on Electrify, Steve Drowne on Skara Mae and Ted Durcan, on Sovrano Dolce, were close behind and were also tangled up in the fall.
Crowley and Tylicki were subsequently taken to hospital in nearby Tooting with suspected spinal injuries.
Drowne and Durcan were able to walk back to the weighing room, while it was reported that the four horses were not seriously injured.
The meet was subsequently abandoned, and course clerk Barney Clifford was visibly shaken by the events.
"The reality is that flat or national hunt racing is a dangerous sport," he said.
"The incident I need to look back again but I guess you don't expect those to happen on the flat and when they do clip heels, that was the cause of the problem and it can be a concertina effect.
"I just hope and pray both Freddy and Jim are ok."