Alpecin, which produces a caffeine-infused shampoo, claimed in adverts that the shampoo “can actually help to reduce hair loss”.

The adverts were headlined: “German engineering for your hair.”

However, the UK’s Advertising Standards Authority has ruled that the claim has “not been substantiated” and is therefore misleading.

The shampoo company had unsuccessfully argued that its adverts implied only a cosmetic effect, not a medicinal one.

It cannot repeat the claim unless it establishes evidence.