Male fertility is being irreversibly damaged by a diet of western junk food by the time men reach 18, a study has found.

A groundbreaking investigation has established that teenagers who favour high-fat and processed foods like pizzas, chips and snacks are killing off sperm-producing cells that can never be replaced.

It showed that a diet dominated by fish, chicken, vegetables and fruit is best is for protecting those cells and ensuring healthy levels of sperm.

Sperm counts have been declining in recent decades and various studies have pointed to poor diet as a significant cause.

However, they have tended to focus on older men - those in their late 20s, 30s and 40s who are struggling to conceive.

By contrast, the new Harvard University investigation shows for the first time that poor diet while growing up may reduce a man’s chances of fathering children for the rest of his life.