LONDON, July 29 (Reuters) - Female circumcision, which is practised in more than 30 countries and affects 2 million girls each year, could cause infertility.

Swedish researchers, who examined nearly 300 women in Sudan where the practice is widespread, said on Friday women who had undergone circumcision, or female genital mutilation (FGM), were five to six times more likely to be infertile.

"All sorts of female circumcision, not only the severe forms, probably cause an increased risk of infertility. This is a very important argument to be used in areas where this is practised," Dr Lars Almroth, a paediatrician and researcher at the Karolinska Institute in Stockholm, said in an interview.

Despite efforts to stop what human rights campaigners have described as an atrocity against womanhood, female circumcision is practised in Africa and is common in some countries in the Middle East. It involves the removal of part or all of the female genitalia.

An estimated 135 million women and girls have been circumcised, according to the human rights group Amnesty International.

It is considered part of the culture, a tradition or a rite of passage to adulthood. In some countries it is viewed as a means of reducing a woman's sexual desire and of safeguarding her fertility.

The research, which is published in The Lancet medical journal, is the first clinical study to show it has the opposite effect.

"We found that the more extensive form of genital mutilation, the higher the risk of primary infertility. The risk is very high -- 5 to 6 times higher -- than in the other group," Almroth said referring to women who had not been not circumcised.

He and his colleagues believe infertility may be caused by infection, inflammation, scarring or by the physical alterations resulting from the circumcision.

In Sudan up to 90 percent of women have had some form of genital mutilation. The average age of circumcision for women in the study was 7 but it is performed on girls as young as 4, according to Almroth.

In some countries crude instruments are used to perform the circumcision and nothing is given to relieve the girl's pain.

The researchers examined 99 infertile women and 180 others who were pregnant for the first time from two hospitals in Khartoum. They controlled for other factors that could cause infertility, such as sexually transmitted infections, age and social and economic conditions.