“If you imagine during the evolutionary time, our chaps would have been strolling across the Savannah with a bit of bearskin tied around their middles, otherwise going commando,” she said.

“Then you put them in a hot bath, or in tight Lycra when they go cycling - it’s not very good for their sperm.”

Professor Pacey said the fertility problems caused by couples waiting later before trying to start families could not simply be solved by men freezing their sperm when they are young.

“You can’t just go to the freezer when you are ready and take out your little sample of sperm and take it home,” he said.

“Every woman in the future would need IVF which in itself has its risks.”

The expert also said the sperm’s quality might diminish during freezing.

“I always say to men, would you rather have a fresh strawberry or a frozen strawberry?” he said.

The academics said the replacement fertility level – the level of fertility required to ensure a population replaces itself in size, had not been met in the UK since the 1970s.