A new male injected contraceptive is almost 96 per cent effective, a trial has found, with the hormone-based jab lowering sperm counts by acting on the pituitary gland in the brain.

But scientists say more work is needed to tackle side effects - including depression, acne, and increased libido - before it is made more widely available.

Over the year-long trial study involving 350 men, couples were initially asked to use both the injections and other birth control methods, before moving on to relying solely on the injections.

Only four pregnancies occurred among the men's partners during that time.Dr Mario Festin, from the World Health Organisation in Geneva, Switzerland, said the findings backed up results from earlier, smaller trials.

The injections contained a long-acting form of progestogen, a hormone which blocks sperm production controlled by the pituitary gland.

Testosterone was added to counter-balance reductions in levels of the male hormone resulting from the treatment.

In 274 of the men taking part, the injection reduced sperm count to one million per millilitre or fewer within 24 weeks. A normal sperm count is between 40 to 300 million.

At the end of the trial, around three-quarters of men said they would be keen to continue with the injections - however, the side effects were so bad for some that 20 men dropped out of the trial altogether.

Researchers stopped enrolling new participants into the study in 2011 due to the rate of side effects reported - with 1,491 incidents reported.

Nearly 39 per cent were found to be unrelated to the treatment, including one suicide and one man who reported an abnormally fast, irregular heartbeat when he stopped receiving the injections.

The results of the study have been reported in the Journal of Clinical Endocrinology & Metabolism, published by the Endocrine Society.