Monthly injections of testosterone lowered sperm count
A male contraceptive jab could be as effective at preventing pregnancies as the female pill or condoms, work shows.
The monthly testosterone injection works by temporarily blocking sperm production and could revolutionise birth control, experts believe.
In trials in China only one man in 100 fathered a child while on the injections, the Journal of Clinical Endocrinology & Metabolism reports.
Six months after stopping the jabs the men's sperm counts returned to normal.
Family planning campaigners welcomed the news and said they hoped an injection would give couples more choice and enable men to take a greater share of the responsibility for contraception.
But experts said more trials were needed to check the safety of the jab.
Previous attempts to develop an effective and convenient male contraceptive have encountered problems over reliability and side effects, such as mood swings and a lowered sex drive.
Despite the injection having no serious side effects, almost a third of the 1,045 men in the two-and-a-half year trial did not complete it and no reason was given for this.
Lead researcher Dr Yi-Qun Gu, from the National Research Institute for Family Planning in Beijing, China, said: "For couples who cannot, or prefer not to use only female-orientated contraception, options have been limited to vasectomy, condom and withdrawal.
"Our study shows a male hormonal contraceptive regimen may be a potential, novel and workable alternative."
He said if further tests proved successful the treatment could become widely available in five years from now.
Mr Laurence Shaw, of the London Bridge Fertility Centre and the British Fertility Society, said: "If a male contraceptive like this became available it would be great and would give people another choice.
"It would empower men to make a decision which involves more than just a condom. At the moment the onus is on the woman and men do not have that much choice.
"But we have been here before with testosterone as a method of contraception. We need more rigorous safety testing."
He said in trials, the female Pill was still more effective as a contraceptive than the new jab, but that in real life they might be comparable because it is easier to forget to take a pill.
A spokeswoman from Marie Stopes International said: "This is a very positive step forward. The more range of contraceptives there are, the better.
"But if this does become available men should still remember that it will not protect them against sexually transmitted infections and they should still use a condom."
Rebecca Findlay of the Family Planning Association said: "In the past fpa has asked men if they would use hormonal contraception, and a third gave a definite yes they would.
"More research is needed to make sure that any new method is safe and effective, but men will welcome the continued search to give them more control over their fertility and sexual health."