It has long been hailed as the cheat's way to exercise - while having fun.

And now Canadian researchers claim to have discovered exactly how many calories the average person burns during sex.

They say that in young people, sex burns an average of 4.2 calories a minute for men and 3.1 calories a minute for women.

With the average session lasting 24.7 minutes, this equates to 104 calories for men and 69 for women.

This makes it better exercise than a walk - but not as good as jogging.

The researchers, from the University of Quebec at Montreal,  looked at 21 heterosexual couples aged between 18 to 35.

During four sex sessions at home, couples wore a SenseWear armband that tracked their energy expenditure. 

They also answered questions on how tired they were afterwards, how much effort they put in and how much they enjoyed it.

Participants had previously undergone a half hour treadmill test so the researches could measure how many calories they burned during exertion - and then compared the readings from the armband worn during sex.

Men, for example, burned 9.2 calories per minute on the treadmill - but only 4.2 in the bedroom.For women, this fell to 7.1 calories while jogging.

And, to help settle another holy grail question, the average recorded session lasted for 24.7 minutes - the shortest being 10 and the longest 57 minutes.

While the figures aren't quite the marathon calorie burn some might hope for, they are better than those suggested in previous research. 

David Allison, a biostatistician at the University of Alabama at Birmingham, found that the average person burns a mere 21 calories during the average session.

He discovered that sex lasted six minutes on average - 'disappointing, isn't it?' - and that the energy output equated to a paltry 20 or so calories - about the same as walking.

However, if done with enough gusto, sex can still count as 'significant exercise', the Quebec researchers said, reporting in the journal PLOS ONE.