A report suggests maternity support workers on wards in NHS hospitals are being given tasks that only fully qualified midwives should do.
Midwives say they are under intense pressure
Researchers at Kings College London say that this may be putting mothers and their babies at risk.
Maternity Support Staff, or maternity care assistants, were first introduced into the NHS in 2005 in response to growing concerns over staffing on maternity wards.
The Royal College of Midwives has warned that the workload faced by midwives is intense, and morale is low.
While the numbers of midwives in training is growing, 20% of all midwives are over 50 years old, and there are fears that many will opt to leave the NHS.
At the moment, more than half of all midwives choose to work only part-time.
The new role was brought in to ease midwife workload by taking some of their duties.
While this varied from hospital to hospital, this could mean anything from clerical work and answering the telephone, to helping out in ante-natal clinics by helping to run parentcraft classes and accompanying midwives on community visits.
If properly trained, the workers could even support women in early labour, carry out blood and urine tests, and make sure women got proper support when learning to breastfeed after having their baby.
Some trusts have given maternity care assistants roles where they help midwives during waterbirths, or help run blood glucose monitoring clinics.
The duties given to the new workers varied from trust to trust, as does the length of training.
After a pilot programme lasting 10 months, the new staff were billed as a success story, beneficial to both midwives and to the experience of new mothers.
In some cases, midwives released up to 64% of their time each week by handing over simple duties to maternity support workers.
The current accusation is that maternity support staff have been told to go further than this - either deliberately or as a result of busy periods on hard-pressed labour wards.
The research says they have carried out tests such as ultrasounds or foetal heart monitors, vaginal examinations or one-to-one care later in labour.
It says that in some cases, maternity support staff have even acted as the sole 'night cover' member of staff at a midwife-led birthing centre.
The report suggested that some trusts, as a cost-saving measure, were closing vacancies for midwives and replacing them with a vacancy for a maternity support worker.
Mervi Jokinen, Practice and Standards Development Adviser for the Royal College of Midwives, said there were certain tasks that it was vital - and legally required - for midwives to carry out.
She said: "You can understand how, when a trust is pushed, and short of midwives, that this boundary could be crossed - but that doesn't make it right.
"There is no excuse for doing this when someone's safety is at risk."
"At a very basic level a maternity care assistant helps carry out all non-clinical tasks on the ward, perhaps helping to feed and clean the women, do paperwork, but we would not expect clinical maternity care to be carried out by them.
"What would be the point of a maternity care assistant carrying out a foetal heart trace if they don't have the skills to interpret it?"
Belinda Phipps, from the National Childbirth Trust, said that all "hands-on" roles during delivery should be left to the midwife, with maternity care assistants hardly coming into contact with them at all during this period.
"What women want is a one-to-one relationship with a skilled midwife while they give birth.
"The last thing they need is to have a room like Piccadilly Circus with lots of people popping in and out to do different things."
The Department of Health has now issued clear guidance on what maternity support workers are not allowed to do.
Chief Nursing Officer Christine Beasley has written to all NHS chief executives reminding them that they have a legal responsibility for all babies to be delivered either by a midwife or a doctor.
In short, they are not allowed to take on any more complex clinical roles - whether being directly involved in late-stage labour or delivery, or monitoring women during labour.