Men who keep a mobile phone in their trouser pocket could be damaging their chances of becoming dads, say scientists.
A team led by Dr Fiona Mathews, of the University of Exeter, conducted a review of the findings from 10 studies, involving 1,492 men.
Dr Mathews said the findings suggest that mobile radiation has an impact on fertility - but said much more research is needed to draw any firm conclusions.
She said: "Given the enormous scale of mobile phone use around the world, the potential role of this environmental exposure needs to be clarified.
"This study strongly suggests that being exposed to radio-frequency electromagnetic radiation from carrying mobiles in trouser pockets negatively affects sperm quality.
"This could be particularly important for men already on the borderline of infertility, and further research is required to determine the full clinical implications for the general population."
The paper, published in the journal Environment International today, found similar effects were seen for sperm viability.
The authors warned that handheld gadgets may be combining with radiation from internet wifi and other technologies to lower fertility rates globally.
They said that 'cumulative' radiation from modern technology may be having a 'cumulative' impact on sperm.
However, Dr Allan Pacey, a leading fertility expert at the University of Sheffield, said he was not convinced by the study.
He said: "In my opinion, the studies undertaken to date have been somewhat limited.
"That's because they have either sperm kept in a dish irradiated at frequencies used by mobile phones - which is not realistic - or they have made assessments of men's phone habits without adequately considering other aspects of their lifestyle.
"What we need are some properly designed epidemiological studies where mobile phone use is considered alongside other lifestyle habits.
"Until that time, I will be continuing to keep my iPhone in my trouser pocket!"