But anti-abortion organisations warned that the effect on the surviving twin and the mother herself could be "horrifying".
In what is believed to be the first "selective termination" of its kind in Britain, Phillip Bennett, professor of obstetrics and gynaecology at Queen Charlotte's Hospital, London, has agreed to terminate one foetus because the mother has said she could not cope with two babies.
Selective termination is usually used when in vitro fertilisation results in a multiple pregnancy. Doctors can choose to abort one of the foetuses if it shows a sign of abnormality, or if a high number of embryos implant in the womb, increasing the risk of complications.
The technique involves piercing the selected foetus with a needle, although this can increase the risk of the others miscarrying. The dead foetus is carried to full term, shrivelling in the womb. In 1994 there were 73 selective terminations, compared with 32 in 1993.
"Killing one healthy twin sounds unethical," Professor Bennett told a Sunday newspaper yesterday. "But my colleague and I concluded this week that it would be better to terminate one pregnancy as soon as possible and leave one alive than to lose two babies."
Dr Vivienne Nathanson, head of ethics for the British Medical Association said the decision was bound to cause "instinctive horror" but could be justified legally on medical grounds because multiple births carry more risks, or on social grounds if the woman could not cope with twins.
"I don't think there's really any difference between performing an abortion to leave no foetus and reducing a twin to a singleton," she said. "It's exactly the same as any other abortion at 16 weeks."
David Paintin, chairman of the Birth Control Trust agreed: "It is a very difficult situation but a termination is the same whether for twins or a single pregnancy. It is only allowed on the grounds listed in the Abortion Act."
He said that if he had been asked he would have carried out the selective termination: "The dilemma is that she says she can cope with one child but not two. If a woman who is 16 weeks pregnant feels so strongly that she cannot cope, one of the options must be selective reduction."
But a spokesman for the Royal College of Obstetricians and Gynaecologists said that he thought most gynaecologists would not be prepared to terminate one foetus only.
Wendy Savage, press officer for Doctors for a Woman's Choice on Abortion, warned that aborting one twin could have severe repercussions for both mother and surviving twin: "If this woman came to me and said that she couldn't cope on the grounds of mental health, I would want to involve a psychiatrist or a psychotherapist," she said. "When you have got twins and one of them is aborted, you have the live twin in front of you as a constant reminder."
Professor John Scarisbrick, chairman of Life, an anti-abortion organisation, described it as a "horrifying story". "What will happen if the surviving twin discovers that a brother or sister is missing and that this is his or her mother's fault? What will this do for the trust and love in the family?" he said.