Faith Williams, the surviving conjoined twin, was gravely ill last night four weeks after her birth and doctors fear that she could die at any time.
The condition of the baby, who was survived an operation to separate her from her sister, Hope, when they were four days old, has shown no improvement since she was put on a ventilator at Great Ormond Street Hospital.
A team of doctors and nurses are keeping her alive, monitoring her condition around the clock.
Laura Williams, 18, and her husband, Aled, 28, are facing an agonising Christmas with their 18-month-old daughter, Carly, having already lost Hope, who died during surgery.
It is four weeks today since the twins were born. A spokesman for Great Ormond Street said last night: "We have been saying quite consistently that Faith is a very sick little girl."
The Williams have a strong religious faith and they accepted that the outcome of the surgery would never be certain.
Faith and Hope were born on November 26 and were joined from the breastbone to the navel. They had separate hearts and lungs but shared a circulatory system, intestine and liver.
Their condition presented an immense challenge to the specialists at Great Ormond Street who described the operation to separate them as one of the most complex they had faced.
Doctors had hoped to separate the girls when they were older but the surgery was brought forward after a blockage developed in their joined intestine.
From their birth, the doctors had feared that problems with their conjoined hearts and circulation systems could require them to have emergency surgery.
The operation took a team of 20 doctors, nurses and theatre staff 11 hours to complete.
Hope, the weaker twin, died when her underdeveloped lungs proved too small to support her breathing. Doctors gave Faith a 50-50 chance.
In the weeks since, Faith has been continuously on the critical list. The condition checks issued by the hospital have been terse and unvarying, stressing that the staff caring for her were doing all they could.
She has had two further operations – to correct a heart defect to improve her circulation and, later, surgery to close her chest which had been left open after the separation surgery.
The twins were born at University College Hospital, London by Caesarean section, and transferred to Great Ormond Street, one of the world's leading centres for surgery on conjoined twins. Mrs Williams made history by becoming the youngest woman to give birth to conjoined twins.
She rejected the offer of a termination when she was told during a routine antenatal scan that the twins were conjoined.
The surgeon, Edward Kiely, who operated on the twins with Professor Angelino Pierro, the leader of the surgical team, is the most experienced in the country and has been involved in the treatment of 16 previous sets of conjoined twins.
Although the operation was a technical success, it was impossible to tell how the twins would fare after separation.
Problems with Faith's heart and circulation have caused doctors the greatest anxiety.
Experience at Great Ormond Street shows that where the separation is performed as an emergency, three out of four babies will die.
When the operation is a planned one, survival rates improve, with eight out of 10 babies leaving hospital alive.
Mr and Mrs Williams said that it was their faith that had kept them going through the pregnancy and the days that followed the birth of their daughters.
"If we didn't have God on our side they wouldn't be here. It was Him who kept them in this world," Mrs Williams said.