'They love, love dearly, both their children and simply cannot bring themselves to choose life for one at this frightful cost to the other'   - Mr Justice Johnson

The Siamese twins "Jodie" and "Mary", born by Caesarean section at St Mary's Hospital, Manchester, this month, pose huge difficulties for the surgeons who will try to save the stronger of the two.

The Siamese twins "Jodie" and "Mary", born by Caesarean section at St Mary's Hospital, Manchester, this month, pose huge difficulties for the surgeons who will try to save the stronger of the two.

The sisters are joined at the lower abdomen and have one heart and a pair of lungs between them, and surgeons will have to reconstruct internal organs and part of the abdominal wall in order to to give Jodie, the stronger of the two, the chance of life.

The surgical team that will carry out the operation, pending any appeal by the Official Solicitor against yesterday's High Court decision, is expected to be led by Mr Alan Dickson and Mr Adrian Bianchi, neonatal surgeons at St Mary's.

Mr Dickson has experience at Great Ormond Street Hospital, London, where he participated in the operation to separate Aoife and Niamh McDonnel, who were born at St Mary's in 1997. They were joined at the liver, which made them easier to separate than many Siamese twins because the liver has the capacity to regrow. Aoife and Niamh, who underwent a six-hour operation, are now healthy three-year-olds living in Ireland.

The High Court's decision to authorise surgery to separate the twins was challenged last night by Dr Richard Nicholson, the editor of the Bulletin of Medical Ethics, who described it as " simplistic".

"The English court's duty is to put the interests of the child first, but there are two children here and it really seems a frankly simplistic decision to say it's all right to kill one to save the other." He added: "It may just be that there are times when perhaps the parents are right and no human institution should be making this choice between babies."

The Liberal Democrat MP Evan Harris, who is a doctor, said: "When there is a disagreement between the babies' doctors and the parents it is more appropriate for these things to be decided by the courts.

"While one sympathises with the parents, I think doctors are quite right not to base life-and-death clinical decisions on the opinions of parents thinking that a particular course of action is 'God's will'."

Siamese twins are identical twins who fail to separate completely during development from a single fertilised egg. The condition affects one in 100,000 births. They are three times more likely to be girls than boys and are more often Indian or African than Chinese or European. If joined by bone or tissue, they are relatively simple to separate. But if internal organs are involved, then the operation is more complex and may result in the death of one or both. In 70 per cent of cases, Siamese twins are joined at the chest. Twins whose heads are joined are the rarest form, accounting for only one in 2.5 million births.

Great Ormond Street Hospital has an international reputation for its work on Siamese twins - 15 sets have been treated there since 1984. Among the 30 children, 12 survived. Professor Lewis Spitz, a professor of paediatric surgery who performed many of the separations, has said that the operation can present unexpected difficulties.

Speaking after successfully separating Italian girls joined at the liver, Professor Spitz said: "The advantage of having done these operations before is phenomenal. The first time, you have no idea what sort of organisational problems you might encounter. Despite the pre-planning, there were still some decisions we had to make on the spot."

Surgeons say that it is best to separate twins as early as medically possible in life. If they live joined for years they can find it difficult to live apart. After twin girls from Thailand were separated in an 11-hour operation at Philadelphia Children's Hospital in the United States in 1995, their foster mother said it was a long time before they realised they could do things on their own.

For others, physical proximity has not prevented them leading their own lives. Yvonne and Yvette McCarthy, who were joined at the head and walked in a crab-like fashion, died in 1993, aged 43. After spending a decade as children touring with a circus in a freak show, they enrolled in a college to study as children's nurses when in their thirties.