The doctor who helped deliver a pair of conjoined twins today told of their parents' heartache when they learned that one must die for the other to survive.

Professor Nicholas Fisk said it was a very traumatic and emotional time for the parents of the baby girls, Natasha and Courtney Smith, who were born yesterday at the Queen Charlotte's and Chelsea hospital in west London.

Both twins will die within a couple of months if they are not separated, because their shared heart is not strong enough for two. The vital organ is located mainly in Natasha's chest, which means that she has been chosen as the one to survive.

Prof Fisk said: "With two babies and one heart, there can be sadly no chance for Courtney of survival."

But Natasha also faces a battle as there is thought to be no example of a baby living more than a few months after such surgery, he added.

He said that the parents, Tina May and Dennis Smith from St Albans, Hertfordshire, had been prepared in advance that at least one of their babies may have to die.

"This was a possibility they have been entertaining for months. It's a very traumatic time for them. Seeing the babies for the first time was quite an emotional moment," he said.

Prof Fisk said the twins were taken to Great Ormond Street hospital shortly after their birth, for detailed examination. An attempted separation operation is expected to happen in around four weeks time.

Today both twins were "stable" and Ms May was "doing well", Prof Fisk said. She was hoping to visit the babies in the next few days, he added.

The twins were born at 10.47am yesterday weighing 4.42kg (9lb 12oz).

They were delivered by a classical Caesarean operation where the incision is made vertically to allow room for both babies to fit through.

As well as having one heart, the twins also share one liver.

Dr Helena Gardiner, a perinatal cardiologist, said: "There are some parts of the circulation we need to investigate further before the surgery can take place."

A spokesperson for the ProLife Alliance said that efforts should be made to see if there was any chance Courtney could also live.

The spokesperson said: "We have always suggested that the possibility of a heart transplant should be considered for the weaker twin. This was the option chosen in a similar case in Italy. Heart transplants in very young babies have been undertaken in the UK.