TOKYO - The secret behind Japan's plunging birth rate? A record 39.7 per cent of Japanese citizens ages 16-49 have not had sex for more than a month -- up five percentage points from two years ago -- according to a survey published this week by the Japan Family Planning Association.

Among married couples, the rate was only slightly lower, at 34.6 per cent.

"This is very bad news for the country's birth rate, and something the government needs to look into urgently," said Dr. Kunio Kitamura, the family planning association's director.

The survey comes amid concerns over Japan's faltering birthrate, which fell in 2005 to a record low of 1.26 births in an average woman's lifetime. The decline has stoked fears of impeding tax revenue shortfalls and labour shortages.

"The situation is dismal," Kitamura said. "My research shows that if you don't have sex for a month, you probably won't for a year."

Kitamura partly blamed stress from busy working lives.

A decline in physical communication skills in an increasingly web-based society was also a factor, he said.

The association said it handed out survey forms to 2,713 randomly selected people, and received responses from 636 men and 773 women in November 2006. It gave no margin of error. In a similar poll taken two years ago, 35 per cent reported having no sex for a month.

Japan came last in a 29-country study of sexual satisfaction published by the University of Chicago last year, with a mere 25.7 per cent of lovers expressing satisfaction in bed.

The country was also in last place among 41 countries in a 2005 poll by condom manufacturer Durex, with people having sex just 45 times a year compared to a global average of 103.