Fast and Furious 9: Vin Diesel's stunt double in coma after serious accident

A stuntman working on Fast and Furious 9 is in a medical coma after a horrific on-set accident. 

Star Vin Diesel was reportedly in "total shock" and "choking back tears" after his stunt double fell 30 feet (9m) onto his head, according to The Sun. 

Watts was reportedly performing a stunt in Diesel's place that involved him leaping from a balcony, when it is feared a safety cable snapped. 

A Hertfordshire Police spokeswoman confirmed that authorities and an air ambulance were called to the Warners Bros Studios following the incident. 

The Sun quotes an unnamed source as claiming Diesel was on set seconds after the accident occured. 

"He looked ashen, totally in shock and blinking back tears. He saw what happened," the source said. 

Watts' fiancee reportedly updated friends and family on his condition in a since-deleted Facebook post.  

"Joe has suffered a serious head injury and is in an induced coma. He is stable and is being monitored closely throughout the night," she wrote. 

"I love him so much and my heart is shattered. He has all his family and friends by his side to get him through this!"

It's not the first time the Fast and Furious franchise has been marred by tragedy - lead actor Paul Walker was killed in an off-set car crash in 2013.