Extra workers drafted in to help hard-pressed midwives could actually be putting mothers and babies at more risk, a report has claimed.
Support workers are supposed to help midwives
Maternity support staff are supposed to free up midwives' time by helping with paperwork and non-clinical duties.
However, Kings College London found some trusts in England try to use them to care for pregnant women, even though they are not sufficiently trained.
Experts stressed support staff should never replace midwives or doctors.
This view was echoed last month by Health Secretary Patricia Hewitt.
The report, commissioned by the Department of Health, found that the training offered to maternity support staff varied widely across England.
But it found managers were enthusiastic about the contribution of support workers to maternity services, saying they freed up midwives to spend more time with women and babies.
New roles included breastfeeding advice and support, outreach services to women in vulnerable situations, running antenatal and postnatal groups, assisting midwives at home births and in birth centres and working in operating theatres.
However, the report found that some trusts had actually removed midwife posts from wards, replacing them with maternity support staff posts.
Some maternity support staff were carrying out tasks which would normally be left for a fully-trained midwife, such as minor examinations.
Professor Jane Sandall, who led the research, said: "There is a danger that support workers could cease to be 'another pair of hands', freeing the midwife and other members of the maternity team from administrative and routine duties in order to look after women.
"Instead they may be called upon to substitute care provided by midwives, without sufficient investment in their training or development. This is a less desirable situation."
Both the Royal College of Gynaecologists and the Royal College of Midwives have frequently raised concerns over low staffing levels on labour wards.
Professor Shaughn O'Brien, vice president of the Royal College of Obstetricians and Gynaecologists, said: "Midwifery units are currently understaffed and so maternity support workers are a major help in providing essential assistance to midwives.
"Implicit in the establishment of midwifery support workers was that they would not replace midwives but they would be additional.
"They were not intended to take on duties for which they are not qualified but are employed to enable trained midwives to provide a high standard of skilled one-to-one care.
"The answer surely must be to provide hospitals with more midwives and consultants to ensure a safe service."
Susanna Cafferty, a spokesman for the Royal College of Midwives, said: "Our members have told us when support staff are being used inappropriately, and obviously we can't support that.
"The Chief Nursing Officer did write to trusts telling them that they didn't want to see maternity support staff used for putting women onto monitors, for example, and we would agree with this completely."
The Department of Health said support workers were undertaking inappropriate tasks in only a minority of NHS trusts.
The Chief Nursing Officer Christine Beasley said it was a legal requirement that every baby must be delivered by a registered midwife or a doctor.
"It is completely unacceptable if a hospital is using a maternity support worker as a substitute for a midwife," she said.
"We are very clear about what the role of a maternity support worker is - to support the maternity team in their day to day duties such as clerical work, supporting women with breastfeeding, increasing access for vulnerable women and enhancing the quality of care."