Elizabeth Hurley says her nephew lost more than four pints of blood after being repeatedly stabbed in a mob attack.
The actress branded the four men who stabbed her nephew, Miles Hurley, 21, on Thursday as "animals" and hoped they will be caught before they hurt or even kill someone.
In an Instagram post, she said: "My 21 year old nephew @mileshurley1 was brutally attacked by four men in London on Thursday night. He was repeatedly stabbed and lost more than four pints of blood.
"A policeman was patrolling close by and administered first aid until the ambulance arrived. The deepest wound just missed severing his spine.
"By some miracle no vital organs were damaged. We are praying that these animals are caught before they maim, or even kill, someone else. These are sad days."
The message appears alongside a photograph of herself and Miles Hurley, who is a model.
She earlier wrote on Twitter: "My 21 year old nephew was repeatedly stabbed in a brutal attack in London on Thursday night.
"The police & hospital are being fantastic. Thank you for all your messages. It is an appalling time for him & our family. The police will be appealing for witnesses."
On the day of the attack, Miles had paid tribute to his aunt in a post celebrating International Women's Day.
He posted a picture of her with another family member, writing: "Love & appreciation for all the strong, happy and independent women around me."
Hurley has previously shared Miles's modelling photos, tweeting: "Here's my handsome nephew."
Another 21-year-old was also stabbed in the attack, police say.