A contraceptive injection for men has been shown to be almost 100 per cent effective, and may also increase libido.

The hormone-based jab is designed to lower sperm counts by acting on the brain's pituitary gland.

Over a year-long trial, nearly 96 per cent of couples relying on the injection to prevent unplanned pregnancies found it to be effective. During this time, only four pregnancies occurred among the men's partners.

However, researchers said more work was needed to address the treatment's reported side effects, which included depression and other mood disorders, muscle pain and acne. However it did also increase libido.

Dr Mario Festin, from the World Health Organisation in Geneva, Switzerland, said: "The study found it is possible to have a hormonal contraceptive for men that reduces the risk of unplanned pregnancies in the partners of men who use it.

"Our findings confirmed the efficacy of this contraceptive method previously seen in small studies."