LONDON, Sept. 13 (AP)--Doctors caring for infant Siamese twins believe the weaker one may be thriving at the expense of her better-developed sister, attorneys told an appeals court panel Wednesday.

The three appeal judges must decide whether to permit the separation of the Siamese twins, in effect condemning one to die.

The parents, who are Roman Catholics from an unidentified Eastern European country, oppose the operation and have appealed a lower court's decision in favor of surgery. They say they wish to let God's will take its course.

The twins, joined at the lower abdomen, were born August 8 at St. Mary's Hospital in Manchester. Doctors said their condition Wednesday was "reasonably stable."

Doctors say both twins will die within six months if they are not separated, but that one twin--identified as Jodie--could survive on her own. The less-developed twin, Mary, could not survive separation, doctors say.

However, Mary recently has been growing faster than her sister, said Adrian Whitfield, a barrister representing the doctors and the Central Manchester Healthcare Trust.

"From the physical point of view, Jodie is not growing, although she is eating well. The surgeon thinks it may be that Mary is drawing nutrition from Jodie and growing at her expense," Whitfield said.

This could influence the timing of surgery to separate the twins, but there was no immediate crisis, he said.

The justices decided Wednesday they would accept written submissions from the Pro-Life Group and the Roman Catholic Archbishop Cormac Murphy-O'Connor.

Murphy-O'Connor, leader of Catholics in England and Wales, publicly supported the parents in a statement on September 5.

"The parents in this case have made clear that they love both their children equally, and cannot consent to one of them being killed to help the other. I believe this moral instinct is right," Murphy-O'Connor said.

"Surely it can only be acceptable for the court to override the rights of parents when there is clear evidence that the parents are not acting in the best interests of their children and wish them harm."

The archbishop added that "it would set a very dangerous precedent to enshrine in English case law that it was ever lawful to kill a person that good may come of it."

Lord Justice Alan Ward, one of three judges hearing the case, said the court had contacted colleagues in Australia, South Africa, and Canada but had found no precedent to guide them.

"There is no comparable authority in any of these jurisdictions so far as they are all aware," Ward said.

Tim Owen, a barrister appointed to represent Jodie's interest, said Mary had no chance of long-term survival, and it was "unreal" to consider Mary's interests separately from those of Jodie.

"Without Jodie, Mary will die. With Mary, Jodie will die," Owen said.

"The purpose of the operation is wholly to maintain life and not to accelerate death by mercy killing or otherwise."