Housewife Miyuki Yanagisawa cannot recall the last time she had sex with her husband. She is certain, though, that their physical estrangement can be measured in years, not months.

While she shares a room with the couple's two young daughters, her husband, a company employee, sleeps alone in another room, grateful for the chance to catch up on his sleep after another tortuously long day at work.

'As long as he is healthy and doing well at work, I can put up with the lack of affection,' Yanagisawa, 44 - who asked that her real name not be used - said of her decade-old marriage. 'Many other women in my age group feel the same. When couples reach a certain age they start calling each other "Mum" and "Dad" - they certainly stop using affectionate nicknames. I think that spells the beginning of the end for sex.'

Yanagisawa is not alone. According to a new report by the World Health Organisation, a quarter of married couples in Japan have not had sex in the past year. The problem worsens with age. While the study found that the 42 per cent of couples in their twenties who had lived together for fewer than five years had sex at least once a week, almost 38 per cent of married couples in their fifties have none.

The Japan Society of Sexual Science deems a marriage 'sexless' when a couple go for a month or more without having sex and does not expect the situation to improve.

People trapped in sexless marriages blame long working hours, a claim backed up by global surveys of sexual activity conducted by Durex. According to the condom maker, Japanese couples have sex 45 times a year, well below the global average of 103 times.

According to one study a fifth of Japanese husbands say they are bored with intercourse, while about 15 per cent say they are simply too tired. A similar proportion of women agree that the spark has gone out of their love lives, although fewer than one in 10 blames their lover's poor performance in bed.

'I expect other people think sexless marriages are strange, but in Japan everything revolves around the children and household finances,' Yanagisawa said. 'Men gradually lose their other influence in a marriage, including the role of sexual partner. My husband and I know that is what has happened in our marriage - but we never talk about it.'