The University of Otago study, reported in the British Journal of Psychiatry, found the risk of mental illness was "proportional to the degree of distress" associated with the abortion.

Prof David Fergusson, of the department of Psychological Medicine, and his team, studied data from women who had been interviewed six times between the ages of 15 and 30, each time being asked whether they had been pregnant and, if so, what the outcome of that pregnancy had been.

More than 85 per cent of women reported a least one negative emotional reaction, including sorrow, sadness, guilt, regret, grief and disappointment. A similar number reported at least one positive reaction, including relief, happiness and satisfaction.

The study found that women who reported at least one negative reaction had rates of mental health problems "approximately 1.4 to 1.8 times higher than women not exposed to abortion".

The report concluded: "Collectively, this evidence raises important questions about the practice of justifying termination of pregnancy on the grounds that this procedure will reduce risks of mental health problems in women having unwanted pregnancy.

"Currently there is no evidence to support the assumptions underlying this practice, and the findings of the present study suggest that abortion may, in fact, increase mental health risks among those women who find seeking and obtaining an abortion a distressing experience."