It is feared a safety cable snapped as Joe leapt off a balcony on set and he suffered serious head injuries as he hit the ground below.
His fiancée Tilly Powell wrote on Facebook that her “heart is shattered” over the devastating news.
She said: “Joe has suffered a serious head injury and is in an induced coma. He is stable and is being monitored closely throughout the night.
“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!”
Filming was halted immediately after the accident and the cast and crew were sent home “fearing the worst” as the stuntman was rushed to Royal London Hospital.
A source said: “The stuntman fell at least 30ft — maybe a bit more. Vin Diesel was seen on set seconds after. He looked ashen, totally in shock and blinking back tears. He saw what happened.”
The unnamed man, who was said to be acting out Diesel’s role on Fast & Furious 9, fell around noon yesterday. Police confirmed they attended and are carrying out inquiries.