Couples are being asked to replace their usual form of birth control with a new male contraceptive in a study to test its effectiveness.
Researchers at The University of Manchester, working in collaboration with nine other centres across the world, will ask men in stable relationships to take part in the trial of the hormonal contraceptive.
The research, which follows a similar trial in China published earlier this month involving testosterone injections, will involve male volunteers aged 18 to 45 being given injections of testosterone along with a second hormone that has been shown to reversibly suppress sperm production.
The combination of two hormones means the trial will require half the frequency of injections as the Chinese study. The two hormones - Norethisterone enantate and Testosterone undecanoate - have already undergone trials to test their safety and were shown to have only mild side-effects in a small number of individuals.
The trial will initially involve up to four courses of injections over six months, during which time the men's sperm count will be measured to ensure it is below fertility levels.
The couples - 60 in Manchester and a further 340 internationally - will then be asked to rely solely on the hormonal method for 12 months while the male partner continues to receive the injections every eight weeks.
At the end of the trial period, the men's sperm count will continue to be monitored to assess how quickly fertility levels return to normal.
Lead researcher Frederick Wu, Professor of Medicine and Endocrinology, said: "There is currently a great imbalance of contraceptive methods between men and women with almost 20 different female methods compared to only condoms and vasectomy for men.
"The World Health Organisation wants to provide more male contraceptive choices - especially reversible methods - to allow couples to better plan their families.
"We know from previous studies that any side-effects are minor, while the risk of pregnancy with this hormonal treatment is similar to that of the female pill and far less than the risks posed by using barrier methods alone.
"Couples taking part in the trial are likely to be married or in long-term relationships and may be looking for alternatives to their existing methods of contraception.''