However, the study by researchers from the University of Geneva (UNIGE) did not find any association between use of the devices and low sperm motility (movement) and morphology (shape).

The research also indicates that where the phone was kept - such as trouser pockets - was not linked to lower concentration and count levels.

However, the number of people who said they did not carry their phone close to their body was too small to draw a firm conclusion on this point.

Various environmental and lifestyle factors have been proposed to explain the decline in semen quality observed over the last 50 years, but the role of electromagnetic radiation emitted by mobile phones has yet to be demonstrated.

Researchers analysed data from 2,886 Swiss men aged 18 to 22, recruited between 2005 and 2018 at six military conscription centres.

They found sperm concentration was significantly higher in the group of men who did not use their phone more than once a week (56.5 million per millilitre), compared with men who used their phone more than 20 times a day (44.5 million per millilitre) - a difference of 21%.

This disparity reduced over the course of the study, suggesting 4G may be less harmful than 2G.

Martin Roosli, associate professor at Swiss Tropical and Public Health Institute (Swiss TPH), said: "This trend corresponds to the transition from 2G to 3G, and then from 3G to 4G, that has led to a reduction in the transmitting power of phones."

Semen quality is determined by the assessment of factors such as sperm concentration, total sperm count, sperm motility and sperm morphology.

Previous studies have shown semen quality has decreased over the last 50 years, with a combination of environmental factors (pesticides, radiation) and lifestyle habits (diet, alcohol, stress, smoking) thought to be contributors.

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Rita Rahban is a senior researcher and teaching assistant in the Department of Genetic Medicine and Development in the Faculty of Medicine at the UNIGE and at the Swiss Centre for Applied Human Toxicology (SCAHT) and is also the first author and co-leader of the research.

She said: "Previous studies evaluating the relationship between the use of mobile phones and semen quality were performed on a relatively small number of individuals, rarely considering lifestyle information, and have been subject to selection bias, as they were recruited in fertility clinics.

"This has led to inconclusive results."

Despite the findings, published in Fertility and Sterility, experts say there is no cause for alarm.

Professor Alison Campbell, chief scientific officer of the Care Fertility Group, said: "This is a fascinating and novel study which should not cause alarm or drastic changes in habits.

"Men looking to conceive, or wanting to improve their sperm health should exercise (but not overheat in their groin area), eat a balanced diet, maintain a healthy weight, avoid smoking and limit alcohol and seek help if they are having problems conceiving."