The conjoined twins who share a heart have only a "very, very small" chance of survival and no immediate attempt will be made to separate them, the doctor in charge of their case said yesterday.
Professor Lewis Spitz said the heart had such abnormalities that an operation to "construct a functioning heart would be extremely difficult, if not impossible".
It means that Natasha and Courtney Smith, who are joined from the navel to the top of the chest and also share a liver could "die fairly quickly", Professor Spitz said.
The twin girls, who were born on Monday to Tina May, 23, and Dennis Smith, 33, are in intensive care at Great Ormond Street Hospital in London. Before their birth, it was hoped that Natasha's life could be saved by performing surgery in which she would gain the heart and the liver, in effect sacrificing Courtney's life. But cardiologists doubt whether the shared heart, which has a substantial hole in one of the chambers, can be adapted to form a single, viable organ for Natasha.
Without such an operation, the heart will be unable to support both twins for much longer. But Professor Spitz, the world expert in the field, ruled out an operation at present because the chances of a successful outcome were "very slim".
If the twins did survive for a few months then surgery may become possible, he said. "The likelihood of survival for both is very, very small but if by some miracle they do survive over the next few months we would be prepared to act."
He would not estimate how long the sisters might live.
Professor Spitz said that at one stage in the mother's pregnancy, the medical team had been optimistic that separation could be attempted. After further scans at 30 weeks, cardiologists said that the outlook was "bleak".
It was now clear that the abnormalities to the heart and its vessels were so complex that repair was "virtually impossible", he said.
"This decision was made on the basis of two things. One is the structure of the heart – to construct a functioning heart would be extremely difficult, if not impossible. Secondly, the coronary blood supply to the heart was such that the muscle of the heart would not have been able to survive following an operation."
Ms May and Mr Smith, from St Albans, Hertfordshire, had been informed of the twins' prognosis and were "very distressed and tearful at that stage", Professor Spitz said. "They were brought over here last night in a hurry because the twins were showing signs of deterioration and we didn't want to bring them over when they were dead or dying. But the babies have come through the night and are reasonably stable."
Conjoined twins occur only in every 100,000 births. In cases where a heart is shared and separation is attempted, there have been no long-term survivors, Professor Spitz said.
The parents, who also have an 11-month-old son, discovered they were expecting conjoined twins in November, but, as a Catholic, Ms May declined to have an abortion.
She said: "My happiness is tinged with the agony of knowing the ordeal that lies ahead for us all."