Men should consider freezing their sperm before having a vasectomy in case they change their minds, as the operation may damage their sperm, scientists warned yesterday.

Vasectomy operations involve surgeons sealing the vas deferens, the tube that transports sperm from the testicles. Nearly 40,000 vasectomies are carried out in Britain each year, although 2,500 are reversed, often when a man wishes to have a family with a new partner. But a study of men who had a reversal operation found that their sperm were 10 times as likely to have genetic defects as sperm from men who had not had a vasectomy.

Researchers tested 21 sperm samples from 18 men and found that 3.3% of their sperm had genetic defects, including abnormally high rates of chromosomal defects called sexual aneuploidies, where sperm had an extra X or Y chromosome. Sexual aneuploidies can give rise to a number of medical conditions in children, such as Klinefelter syndrome, in which boys are born with an extra X chromosome and often develop abnormally proportioned bodies and experience learning difficulties.

A rarer condition called Triple X syndrome, affects around one in 2,000 girls, and is caused by sperm carrying two X chromosomes instead of the usual one. It can lead to women developing symptoms including seizures and infertility.

Professor Nares Sukchareon of Chulalongkorn University, Bangkok, who led the study, said that he believed a pressure build-up in the vas deferens disrupted the normal production of sperm in men who had vasectomies. "The conclusion of the study is that the vasectomy seemed to be the cause of the abnormal sperm," he told the European Society of Human Reproduction and Embryology yesterday.

He advised men to have samples of their sperm frozen before having a vasectomy, so they have healthy sperm available should they decide to have more children. He added that if a couple went on to conceive naturally after the man had a vasectomy reversal, the woman's body might select only the best sperm. But if the sperm were used in artificial fertilisation, the genetic defects might be passed on to the child.

"We need a lot more evidence and research on this issue before we can be certain of avoiding the dangers," he said.