Scientists believe they may have found a safe and more effective way of increasing the size of a man's penis. Researchers in the United States carried out experiments in which they successfully carried out penis implants on rabbits.

The scientists grew penis tissue in the laboratory which was then surgically implanted into the rabbits. The rabbits were able to copulate, penetrate and produce sperm.

The discovery raises hopes of improved treatments for infants born with ambiguous genitalia or those in need of penis reconstruction after an accident. It could also one day help those men who want a larger penis.

Dr Anthony Atala and colleagues at Harvard Medical School extracted samples from the penis muscle and erectile tissue of rabbits. These cells were grown in the laboratory and within days resembled real erectile tissue. The scientists then removed erectile tissue from the penises of 18 rabbits, replacing it with the engineered tissue.

Because the tissues were grown from the rabbits' own cells, there was no problem with immune rejection. The rabbits were then placed in a cage with a female rabbit. The males had full sexual functions. "They were able to copulate, penetrate and produce sperm," Dr Atala told New Scientist magazine.

Human tests are 'far off.' The scientists are now trying to grow an entire penis from scratch. It is one of the most difficult organs to grow in the laboratory because it is so complex. However, the scientists have warned that they are a long way off being able to test the technique in humans.

They have also said that the procedure could not work for those women hoping to become men. This is because they do not have the right cells. But it could provide an alternative treatment in the future. At present, the penis is enlarged by injecting fat cells or 'pulling out' that part of the organ that is inside the body. These methods are regarded as crude by some and are not always successful.

The study was originally published in the Journal of Urology.