The scientists believe younger men were most affected by cannabis simply because they were more likely to use greater quantities.

Chemicals in the drug itself, not the tobacco used in ‘joints’, were probably responsible since cigarette smoking had little effect on sperm.

One of the team leaders, Dr Allan Pacey, senior lecturer in andrology – the study of male health – at Sheffield University, said habitual cannabis users who wanted to become fathers should kick the habit for at least three months.

‘I can’t tell you definitively that your sperm will improve, but that’s a reasonable assumption,’ he said.

He said the summer influence was surprising but it did not appear to be linked to heat or significantly alter seasonal conception patterns.

‘We didn’t expect this,’ he said. ‘It’s complete conjecture on my part, but it could be a sunlight effect. Day-length affects our hormones and  vitamin D in all sorts of ways.’ 

He added that men wanting to be fathers should not wait until winter.

However, the study, published in the journal Human Reproduction, found that abstaining from sexual activity for more than six days improved sperm quality.

It was possible that unmeasured factors, such as the quality of sperm DNA, influenced the results, said the researchers.