Scientists at the King's College in London are developing a new contraceptive drug which they hope could prevent men from ejaculating sperm during sex.

The researchers noticed over a decade ago that blood pressure and schizophrenia drugs had the same effect, and have identified chemicals which can block the male sperm.

But these drugs have side effects such as dizziness and drowsiness, which meant they could not be used as contraceptives.

The team is planning to test the chemicals in animals and humans and hope to have a treatment in five years.

This treatment means couples could share contraceptive responsibility.

Several other male contraceptives, given as injections, implants or patches are under development, but these ones are based on hormones that trick the brain into switching off hormone production.

The new treatment acts by preventing the longitudinal muscle in the vas deferens contracting to propel sperm with sperm cells into the seminal liquid.

The team is going to test the treatment on animals first and then on humans.

Men would take a pill each day, as women do with the female contraceptive pill, or a few hours before having sex.

As the contraceptive is not hormonal, a man's fertility should return the following day.

"If a 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." said Dr. Christopher Smith.

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

"For women, it would be another form of liberation."

Dr Allan Pacey, honorary secretary of the British Fertility Society, said: "I would welcome the concept, if further tests showed it to work."

"There is a need for something that men can take."

He was concerned that sperm would be "redirected" into urine, or be present in the urethra, and that this way, the method would not be 100 % secure.