Researchers in Japan are worried that a lack of married couples having sex could be a growing problem for the county's birth rate.

A study, conducted by Nihon University's Population Research Institute and the World Health Organisation (WHO), found that one-quarter of married couples had had no sex in the past year, while 37.3 per cent of those in their 50s were not having sex.

Those most likely to be having sex were young married people in their 20s who had lived together for less than five years, with 42.2 per cent saying they had sex at least once a week or more often.

"There may be a need to look into the fundamental aspect of reproduction - the frequency of having sex - as a new problem in a nation with a low birth rate," said the researchers.

A conference, to be attended by experts from the WHO, the institute and the International Union for the Scientific Study of Population, will be held in Tokyo this November to discuss the issue.

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