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British scientists have developed a revolutionary pill that men could take as a one-off contraceptive just before a date.

The tablet would prevent a man from being able to impregnate a woman, but within a few hours his fertility would return to normal.

This would make it much more acceptable to men than other 'male pills' under development, which alter hormone levels and have to be taken over the long term.

It is also more likely to be trusted by women as they are not relying on their man having to remember to take his pill every day for it to work.

The hormone-free 'male pill' was inspired by two medicines already in use and so the scientists hope it could be on the market within as little as five years.

Experts believe it could transform family planning by allowing couples to share the responsibility for contraception - a role that traditionally falls to women.

The new contraceptive is likely to appeal to women who are uneasy about the female Pill's ability to raise the risk of strokes, heart attacks and potentially-fatal blood clots.

Critics argue, that men lack women's motivation to prevent pregnancy, making it hard for women to trust them to take a contraceptive pill.

Other male pills are under development but many of them are based on hormones that trick the brain into switching off sperm production.

These are typically being developed as injections, implants and patches.

However the new pill being researched by scientists at King's College London, contains chemicals that prevent ejaculation and could be in tablet-form.

Men could take one daily, just like the female pill, or have one a few hours before sex as a one-off contraceptive.

Sexual satisfaction is not affected and the absence of hormones means that a man's fertility should return to normal within hours of stopping the treatment.

Researcher Dr Nnaemeka Amobi said: "The non-hormonal male pill could be taken when and as needed."

Fellow researcher Dr Christopher Smith said: "If the man was taking the pill over a period of several months and decided to come off it, we would expect his fertility to return just as quickly as if he had taken it on a one-off basis."

The contraceptive was inspired by the observation that some drugs used to treat schizophrenia and high blood pressure also prevent ejaculation.

However, side-effects including dizziness and drowsiness mean these medicines could not be marketed as contraceptives.

After pinning down how the drugs stop ejaculation, the London researchers set about creating tablets that do the same thing but without the side-effects.

Already tested in the lab, it is hoped human trials will start shortly and the pill on the market within the next five years.

Currently, men who want to take responsibility for contraception have limited choice, with their options extending to condoms, a vasectomy, or simple abstinence.

Professor John Guillebaud, one of Britain's leading experts on contraception, described the pill as "a brilliant discovery".

He said its strength lay in its ability to prevent pregnancy without using hormones which could cause side-effects such as hot flushes and moodiness.

If the male pill is successful it could bring in huge amounts of money to King¡¯s College, which owns the rights to the discovery.

Annual world-wide sales of the female Pill are worth ¡ê21billion a year.

Rebecca Findlay, of the Family Planning Association, said: "It gets really tiring for women to always be the one in charge of fertility.

"For women, it would be another form of liberation. It's great."