One-quarter of married couples in Japan have had no sex in the past year, a survey shows, urging an open discussion of the issue as the country struggles to boost a dwindling birth rate.

Sex is particularly elusive as people grow older, with the study finding that 37.3 per cent of Japanese married couples in their 50s were not having sex.

Nihon University's Population Research Institute, which conducted the study with the World Health Organisation (WHO), said that there needed to be a serious look at the issue.

"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," the institute said in this week's report.

There was no comparable data for other countries but earlier surveys by condom manufacturer Durex has put Japan among the world's least sexually active nations.

Japan has one of the world's lowest birth rates, presenting a potential future demographic crisis for the world's second largest economy which takes in few immigrants.

The Government has been studying ways to make society more family friendly, including providing more legal protection to professional mothers and reducing Japan's notoriously long working hours.

But Naohiro Ogawa, head of the institute, said that Japan's low birth rate cannot be explained just by socioeconomic factors, saying the study found no correlation between sex drive and stress.

Experts from the WHO, the institute and the International Union for the Scientific Study of Population, will hold a conference in Tokyo in November to discuss the matter, he said.

Sex was most frequent among young married people.

The survey found that 42.2 per cent of couples who are in their 20s and have lived together for less than five years had sex once a week or more often.