The investigation, by the Confidential Enquiry into Maternal and Child Health (CEMACH), comes amid growing unease among clinicians over a legal ambiguity that could see them being charged with infanticide.

The Royal College of Obstetricians and Gynaecologists, which regulates methods of abortion, has also mounted its own investigation.

Its guidelines say that babies aborted after more than 21 weeks and six days of gestation should have their hearts stopped by an injection of potassium chloride before being delivered. In practice, few doctors are willing or able to perform the delicate procedure.

For the abortion of younger foetuses, labour is induced by drugs in the expectation that the infant will not survive the birth process. Guidelines say that doctors should ensure that the drugs they use prevent such babies being alive at birth.

In practice, according to Stuart Campbell, former professor of obstetrics and gynaecology at St George’s hospital, London, a number do survive.

“They can be born breathing and crying at 19 weeks’ gestation,” he said. “I am not anti-abortion, but as far as I am concerned this is sub-standard medicine.”

The number of terminations carried out in the 18th week of pregnancy or later has risen from 5,166 in 1994 to 7,432 last year. Prenatal diagnosis for conditions such as Down’s syndrome is increasing and foetuses with the condition are routinely aborted, even though many might be capable of leading fulfilling lives. In the past decade, doctors’ skill in saving the lives of premature babies has improved radically: at least 70%-80% of babies in their 23rd or 24th week of gestation now survive long-term.

Abortion on demand is allowed in Britain up to 24 weeks — more than halfway through a normal pregnancy and the highest legal limit for such terminations in Europe. France and Germany permit “social” abortions only up to the 10th and 12th weeks respectively.

Doctors are increasingly uneasy about aborting babies who could be born alive. “If viability is the basis on which they set the 24-week limit for abortion, then the simplest answer is to change the law and reduce the upper limit to 18 weeks,” said Campbell, who last year published a book showing images of foetuses’ facial expressions and “walking” movements taken with a form of 3-D ultrasound.

The Department of Health was alerted three months ago to the issue of babies surviving failed terminations. In August clinicians in Manchester published an analysis of 31 such babies born in northwest England between 1996 and 2001.

“If a baby is born alive following a failed abortion and then dies (because of lack of care), you could potentially be charged with murder,” said Shantala Vadeyar, a consultant obstetrician at South Manchester University Hospitals NHS Trust, who led the study.

A systematic investigation of data collected through the CEMACH indicated that there are at least 50 cases a year nationwide in which babies survive abortion attempts.

“First sight of our data suggests this is happening,” said Shona Golightly, the agency’s research director. She said official confirmation of the figures would be available next year.

It is not known how many babies who survive attempted abortions go on to live into adulthood.