'Not tonight, Joséphine.' Napoleon Bonaparte's lacklustre response to the bedtime blandishments of his wife is being repeated every evening in bedrooms across the country. Men are simply going off sex, according to the UK's largest firm of relationship counsellors.

Relate, which provides counselling, sex therapy and relationship education, said there had been a 40 per cent increase in male clients admitting that, despite being physically able to have sex, they can't be bothered.

'Men used to come to us with impotence - now known as erectile insufficiency - but Viagra has sorted some of that problem,' said Peter Bell, Relate's head of practice. 'What we have is a lot of men who say, as women did in the Fifties: "I can have sex, but I don't want to. It's not rewarding".'

Bell says that around half the men he is now seeing admit to a complete lack of libido. Ten years ago, he said, such complaints were unheard of.

'They tend to be men in their thirties, forties and fifties and married,' he said. 'It is a serious issue. It counts as a psychosexual dysfunction, rather than just a relationship problem, because these men haven't simply gone off their partner, but off sex altogether.'

Billy, 43, a graphic designer in Newcastle upon Tyne, who has been married for 10 years, said his sexual desire had waned dramatically in the past three years. 'I still love my wife and am very clear that she's the woman I want to spend the rest of my life with, but I just don't want to make love with her any more,' he said. 'The curious thing is that I can get erections, and I don't fancy or fantasise about other women. It's just that, over the years, my desire to have sex with anyone at all has faded.

'I'm not particularly unhappy about the situation, but I am curious and have tried to work out why I feel this way,' he said. 'My wife and I have always had a difficult relationship but, in the past, that always led to great sex. It's true that I have a stressful job, but previously sex helped me relax and forget my work problems. I'm baffled by my lack of interest, but not particularly unhappy.'

Bell said the problem is 'partly because women are more aware of what they want sexually and are prepared to ask for it'. He added: 'I think it's also that men and women are more sexually similar than they like to think. It is traditionally believed that, while women only enjoyed sex if it happened in the context of a positive and nurturing relationship, men could always be turned on by visual cues alone. But what we're seeing is that, once the thrill of the chase has disappeared and the sex is happening in a committed relationship, the libido of both men and women is affected by the quality of the relationship they are in.'

Professor Michael King, of the Royal Free and University College Medical School in London, believes the problem might lie in depression. Last week he completed a study into mental illness across six countries which found that the rate of major depression and panic syndrome was highest among men in the UK. 'Men are most likely to suffer depression between the ages of 30 and 50,' said King. 'One of the explanations is that men are less able to talk about their problems than women or express their emotions.'

Professor Cary Cooper, president of the British Association of Counselling and Psychotherapy, agreed. 'Men have less social support and, as a generalisation, are less emotionally intelligent than women and have not traditionally been encouraged to share their feelings,' he said.

Cooper, who is professor of organisational psychology and health at Lancaster University, also blamed Britain's culture of long working hours. 'Britain's work culture has gone from 9 to 5 to extremely long hours, which makes for a very stressful life,' he said. 'Stress can be cumulative, which means eventually people can find it impossible to switch off and relax.'