Italian prosecutors have opened an investigation into a botched selective abortion that the Vatican has described as the result of a "culture of perfection" resembling Nazi eugenics.

The deeply Catholic country was embroiled in a bitter ethical dispute yesterday after it emerged that a surgeon had accidentally terminated a healthy foetus instead of its twin with Down's syndrome. The operation - on a 38-year-old woman 18 weeks into her pregnancy - was performed at the San Paolo hospital in Milan in June but has only now come to light. The foetus with Down's syndrome was also aborted subsequently.

The revelation has reignited the debate in Italy over abortion, which was legalised only in 1978. The law allows terminations of healthy foetuses up to the 90th day of pregnancy, though abortions can be performed at a later stage if there is a risk to the life of the mother or the foetus is malformed.

Anna Maria Marconi, the gynaecologist who carried out the Milan abortion, said that the woman - who has not been named - requested the operation after an amniocentesis test.

Professor Marconi said that her conscience was clear. The foetuses, which had been identical, had changed positions in the womb between the last scan and the operation, an "act of fate that could not have been foreseen", she said. The professor was backed by the hospital authorities.

The mother, who has a small son, said that her life had been ruined. "Neither my husband nor I can sleep at night," she told the Corriere della Sera, which first reported the blunder. She said that the happiness she and her husband had experienced when they learnt that she was expecting twins had been transformed into heartbreak.

Her husband said that they were "truly desperate over this terrible mistake" and were consulting family lawyers.

The episode has been seized upon by Catholics campaigning to have the abortion law repealed. L'Osservatore Romano, the Vatican newspaper, said: "No one has the right to suppress another life and take the place of God for any motive whatever." The paper said that selective abortion amounted to eugenics, stemming from "a culture of perfection". The Association of Catholic Doctors said that selective abortion was "the fruit of an egoistic culture", while Luca Volontè, a Christian Democrat politician, said that the Milan mistake amounted to infanticide.

However, Carlo Flamigni, a leading fertility surgeon and a non-Catholic, argued that repealing the abortion law would be like "closing down all motorways just because now and then someone causes an accident by falling asleep at the wheel". Rita Bernardini, of the pro-abortion Radical Party, said that if the "pro-life" campaign succeeded, prospective parents could lose the right not only to abortion but also to prenatal diagnosis warning them of birth defects.

Giorgio Bolis, the head of obstetrics and gynaecology at the San Paolo hospital, said that such cases were "very rare indeed" and that an internal hospital inquiry had shown that there were "no procedural errors". Livia Turco, the Health Minister, insisted that the existing abortion law was "very wise" and would not be altered.

According to the Health Ministry, the number of annual abortions in Italy dropped from a height of 234,801 in 1982 to 129,588 in 2005.