Most women enjoy best sex as they approach 40 because they fear that their "biological clock" is ticking, say researchers.

A new study has found women are more likely to have sexual fantasies and affairs in their 30s and early 40s than in their younger years, despite the fact their bodies may be in decline.

According to the researchers, this is because as women approach the milestone they may sense that their "window of opportunity" to have kids is closing and their fertility is declining, 'The Daily Telegraph' reported.

And, this instinctual reaction is an increased appetite for sex, said the researchers who have based their findings on a survey of 900 women.

The survey divided respondents into three groups – those at their most fertile (aged 19 to 26), those whose fertility was declining (aged 27 to 45) and those who were approaching or had reached the menopause.