A third of over-50s are having an affair

But nearly a third claim to be having affairs instead.

Older people are much more likely to be tempted into infidelity than the young, a study has found.

The reason, according to an examination of more than 13,000 sex lives, is that those in their 20s and 30s are not likely to have settled into marriage or a long-term relationship.

And even if they have, they are more likely to be in "the first flush of romance".

But the middle-aged find the lure of an affair "overwhelming", according to the survey results.

After the hard work of maintaining a marriage and often a family for so long, perhaps they can't resist what they regard as a last chance for a little self-fulfilment.

And it seems the general lessening of sex drive after 50 is no barrier to adultery.

In some cases, it is in fact the final straw that causes a husband or wife to seek solace with a more accommodating lover.

Psychotherapist Brett Kahr, who led the British Sexual Fantasy Research Project, said: "I would be hard pressed to recall any couple who presented for marital psychotherapy with a healthy sex life.

"As we may not fully appreciate, sex might be the most sensitive barometer of the solidity of the relationship between husband and wife, or between two lovers.

"When the gremlins of infidelity or inattentiveness or other forms of cruelty enter the relationship, then the sexual life will suffer as a consequence."

The survey, which asked for detailed information on sex lives, was sent to more than 34,000 people.

Just over 13,000 replied. The findings showed that 14 per cent of those under 30 had had sex with someone outside their marriage or long-term relationship, as had 23 per cent of those between 30 and 40.

Appetite for sex tails off after 50, the survey found. The number of over-50s who claim to have sex once a day or more was too small to record.

Nineteen per cent had sex three times a week, 44 per cent between twice a week and once a month, and 32 per cent less than once a month.

Just over eight million were not having sex at all, the researchers said. Fewer than a million have always been celibate – the others have simply given up sex.

More women than men, 21 per cent against 15 per cent, have had sex in the past but have now stopped.

Despite the fling-prone over 50s, the great majority of all those in relationships are faithful most of the time: men have on average 1.18 sexual partners in a year, while women have 0.7 partners.

The survey also undermined a regular claim of the gay lobby that between 5-10 per cent of the population are homosexual. Just three in 100 said they were gay.

Infidelty by numbers

Thirty-one million Britons are married or live with a partner.

11 million of these have had an extramarital kiss.

42 per cent of men have kissed another person while in a relationship.

31 percent of women have kissed another person while in a relationship.

14 per cent of under-30s have had sex outside marriage or partnership.

30 per cent of over-50s have had sex outside marriage or partnership.

23 per cent of 30 to 40 year olds have had sex outside marriage or partnership.

15.64 is the average number of women a man has sexual contact with in his lifetime.

14.56 is the average number of men a woman has sexual contact with in her lifetime.