Snatch and sniff: The best way to find your perfect partner could be by using your nose - but the Pill might disrupt your sense

They say that opposites attract. But it seems the Pill may be preventing women sniffing out men who are opposite enough.

Women are said to have an inbuilt ability to pick up the scent of a partner who differs genetically.

Falling for this type of man helps ensure that the couple's children will have broad immunity against disease, so the theory goes.

But researchers found that the Pill disrupts a woman's power to recognise the aroma of a suitable partner.

Saddled with the wrong man - someone who in scientific terms has similar genes - she may find it hard to become pregnant and any children she does have may have a lower resistance to infection.

What is more, when she stops taking the Pill and her sense of smell returns to normal, she's more likely to fall out of love, the Liverpool and Newcastle universities research suggests.

It is thought that women subconsciously use the smell of a man's sweat as a guide to the genetic make-up of his immune system.

But this research shows that the Pill sends the rules of attraction into meltdown by making women set their sights on men similar to themselves.

To measure its effect, scientists asked a group of men to sleep in T-shirts and steer clear of deodorants and other fragranced products.

The T-shirts were frozen until needed, then defrosted and placed in glass jars with 'nose holes' in the lids.

Almost 100 women then sniffed the shirts and gave their opinions on the 'pleasantness' and ' desirability' of the odour twice over a three-month period. Many started taking the Pill during the experiment - and their opinions of the smell of the T-shirts changed, the journal Proceedings of the Royal Society B reports.

Researcher Craig Roberts said: 'The results showed that the preferences of women who began using the contraceptive Pill shifted towards men with genetically similar odours.'

Previous studies have shown infertile couples tend to have more similar immune systems than othersand that similarity can lead to a roving eye. Dr Roberts said the decision to go on the Pill could have far-reaching consequences.

'Although smell is a small thing, it is a fundamental thing. If you really don't like someone's smell, it could lead to the odd argument and you might be more likely to seek out affairs.'

It is thought the body confuses the hormonal changes caused by the Pill with those of pregnancy. In pregnancy, an altered sense of smell may help a woman bond with relatives who will help her raise her baby.

But fertility doctors urged women not to throw away the Pill.

Professor Bill Ledger, of Sheffield University, said: 'Whilst a relationship is going, I would think the intellectual and emotional feeling you have for that person would surely override what their sweat smells like.'