The BBC's Bob Sinkinson on the twins' current condition

Surgeons on Thursday said the twins, who were airlifted to Great Ormond Street Hospital for the life-saving operation last week, had a better than 50/50 chance of survival.

Their health is continuing to improve, but they are on ventilators and need intensive medical care.

The two surgeons, Professor Lewis Spitz and Mr Edward Kiely, said the operation to separate them had gone better than expected, although they had not been particularly well after the operation.

However, they had since rallied and were in a stable but critical condition.

Another operation

Each weighs 2kgs. After the operation, they did not have enough skin to cover their abdomens so the area has been covered with a plastic mesh which will help knit the skin and muscle together.

An operation to removed the mesh could be undertaken in the next 48 hours.

Mr Kiely said: "Their tummies are wide open, but once the hole is closed it will heal like any other operational site."

The twins need 24-hour care and, even if they make a suitable recovery, they are likely to be in hospital for at least three months.

It will not be until next spring that they can be taken off intravenous feeding, a nurse said.

The surgeons praised the RAF, doctors in Bristol, where the twins were born, and doctors in Gloucester who gave their parents pre-natal advice for their actions to save them.

They said the twins' condition is not expected to change in the next few days.

Their parents are reported to be bearing up under the strain and may be able to handle the girls by next week.

Life-saving operation

During last week's three-hour operation to save the twin girls, one needed external cardiac massage to restart her heart.

Mr Kiely sat with the baby through the night and gave her tiny doses of drugs to keep her alive.

Usually, Siamese twins are not separated until they are about three months old because they are stronger then to withstand the operation.

However, these twins were an urgent case. They were born four weeks early by Caesarian and were suffering from peritonitis.

Both have damaged and infected intestines and the stronger child has had an operation to repair this.

The other girl will have a similar operation when she is strong enough.

Great Ormond Street Hospital where the twins were operated on last week

Describing last week's operation, Professor Spitz said: "Their livers are like soft butter and controlling the division and controlling the bleeding was a major difficulty."

He added that the separation operation could just be the start of their health problems.

They are at high risk of infection and the intravenous feeding could cause problems.

"Obstruction and jaundice can be a problem and again getting themto feed on their own may also be a difficulty," said Professor Spitz.

One in 200,000

The chances of having Siamese twins are rare.

One in 50,000 babies is a Siamese twin, but 60% are stillborn, leaving only 200,000 who survive on birth.

Twins are three times more likely to be girls than boys.

Professor Spitz and Mr Kiely have operated on 14 Siamese twins since 1984.

Seven pairs have died and in three cases, one of the twins has survived.

Only one set of surviving twins - Niamh and Aoife Varley - is British.

The girls, born last April, were joined at the chest and the abdomen and shared a liver and pericardial sac - a membrane which protects the heart and major blood vessels.