Prince William will be completely bald on top of his head by the time he is 40 if he takes no action, according to a leading hair transplant surgeon.

Doctor Asim Shahmalak said that Propecia, which Justin Bieber suggested that William should use, could halt the royal’s hair loss but it would not restore the lost hair.

“He would have to take Propecia for the rest of his life and it would not help him to grow new hair,” the Daily Mail quoted Shahmalak, who runs the Crown Clinic in Manchester, as saying.

“Around 2 per cent of patients who take Propecia suffer side effects such as the loss of libido,” the doctor said.

Shahmalak, who carried out a hair transplant on Dr Christian Jessen from Channel 4’s ‘Embarrassing Bodies,’ said that the 30-year-old Duke of Cambridge need to take action as soon as possible.

“Prince William has been losing his hair since he was 25. His hair loss is even worse than his father Charles. And obviously his grand-father Philip is bald too.

“That is a strong family gene. There is a strong likelihood that any male child he has with Kate will also have this bald gene - though a further consideration will be hair genes on Kate’s side of the family.

“Prince William has quite fine light brown hair. To restore the hair on the top of his head to a reasonable level of density, he will need at least 3,000 or 4,000 hair grafts where hair is taken from the back of the scalp and moved to the top of the head. This is similar to the treatment which Wayne Rooney had,” he said.