The doctor leading the treatment of conjoined twins who share a single heart confirmed yesterday that one of the babies must die for the other to survive.

Professor Nicholas Fisk said the girls, Natasha and Courtney Smith, who were born joined at the chest on Monday, shared one heart which could not support both infants for long.

Speaking at Great Ormond Street Hospital in London, where the girls are being treated, Professor Fisk said Natasha would be given the heart as it was further into her body. He said the inevitable result would be the death of Courtney and that even for her sister the odds of surviving such traumatic surgery were slim.

The parents, Tina May and Dennis Smith, from St Albans, Hertfordshire, have known for some time that their twins were conjoined and that one of the babies might have to die.

The babies, who were born at the Queen Charlotte's and Chelsea Hospital in west London, will be assessed and examined before surgery at Great Ormond Street in the coming weeks. Doctors said both twins were stable and needed no artificial help to breathe or function. Their mother is expected to be discharged from hospital by the end of the week.

An anti-abortion group, the Pro-Life Alliance, called for doctors to give Courtney a heart transplant if possible.

But Prof Fisk underlined the difficulty of that, saying: "With two babies and one heart there can be sadly no chance of survival for Courtney. For Natasha, she will succumb within a year at the latest, unless the abnormality in her heart is corrected. That cannot be corrected while the circulation of Courtney is hooked onto her own."