An injected male contraceptive which lowers sperm counts to prevent unplanned pregnancies was effective for 96% of couples during a year-long trial.

The hormone-based jab was used by 350 men and their partners, and only four pregnancies occurred.

However, there was a significant number of reported side effects, which forced scientists to stop enrolling new participants into the study five years ago.

They included depression, muscle pain, acne and increased libido - and caused 20 men to drop out of the trial.

Of the 1,491 incidents reported during the study, 39% were found to be unrelated to the treatment. They included one suicide.

One man experienced an unusually fast and irregular heartbeat when he stopped receiving the injections, which were given every two months.

The jabs contained a long-acting form of progestogen, a hormone which works to block sperm production controlled by the brain's pituitary gland.

Testosterone was also added to counter-balance reductions in male hormone levels.

Three in four of the men who completed the trial said they would be willing to continue using the contraceptive injection.

Dr Mario Festin, from the World Health Organisation, said: "More research is needed to advance this concept to the point that it can be made widely available to men as a method of contraception.

"Although the injections were effective in reducing the rate of pregnancy, the combination of hormones needs to be studied more to consider a good balance between efficacy and safety."

When used correctly, condoms are said to be 98% effective, and the pill more than 99%.