The study found the number of healthcare assistants is rising at four times that of nurses, amid a shortage of around 35,000 nurses.
For every nurse taken on between December 2015 and December 2017, four of the cheaper workers were hired.
In some areas, hospitals lost almost exactly as many nurses as they hired healthcare assistants, raising fears that the cheaper workers were being used as substitutes, endangering patient safety.
The study found almost one in three care roles in NHS trusts is now filled by a healthcare assistant.
Patients’ groups said the trends were “frightening”.
It follows warnings that the workers are being used as “nurses on the cheap” and left unsupervised because of shortages of nurses.
Healthcare assistants are meant to help trained nurses, carrying out basic tasks such as feeding patients, answering calls bells and helping them to the toilet.
But there is increasing concern that they are being used to perform tasks for which they are not trained - such as administration of medicines, or being left in charge of multiple patients.
The study, by BPP University School of Nursing, says soaring vacancies of nurses over the period paint an "alarming picture,” with heavy reliance on agency nurses, as well as assistants.
Worst affected areas are the north west and east of England, where nursing vacancies rose by 48 per cent and 46 per cent respectively.
Across all English trusts, the nursing and midwifery vacancy rate rose from 28,713 in December 2015 to 34,682 in December 2017 – a rise of 21 per cent they wrote.
"Although nursing numbers appear to be holding up, the sharp increase in vacancies tells a different story. They have soared by over a fifth in two years," they wrote.
"To plug that care gap, trusts are increasingly turning to healthcare assistants, whose numbers have grown substantially."
Healthcare assistants, whose pay starts at around £17,000 a year, have far less training than nurses, who are required to undertake a nursing degree, with starting salaries of around £23,000.
The study warns: “Although nursing numbers appear to be holding up, the sharp increase in vacancies tells a different story. They have soared by over a fifth in two years – although there are significant regional variations. As the country’s ageing population has risen, demand for more care and more complex care has increased. There are 2,200 more emergency admissions per day than there were five years ago, for instance, 31 per cent more diagnostic tests, while delays due to waiting for available home care have more than doubled.
“Unfortunately, supply hasn’t increased in line with that demand – hence the shortfall in recruitment and the rise in use of agency nurses, which increased by 37 per cent between December 2015 and December 2017.”
Overall, while the number of nurses rose by 0.5 per cent between December 2015 to December 2017, the number of healthcare assistants rose by 6.5 per cent, the analysis shows.
In total, 29 per cent of care roles in NHS trusts in England are now being carried out by healthcare assistants, the study found.
The report, was based on responses to Freedom on Information requests to NHS trusts in England – of which 67 per cent of acute trusts responded.
Joyce Robins, from Patient Concern, said the trends were “frightening”.
She said: “This is really worrying, there is such a paucity of trained staff, and we are seeing far too much reliance on staff who just aren’t trained for the tasks they are doing.”
Dr Anne Corrin, head of professional learning development at the Royal College of Nursing, said: "Healthcare assistants offer vital support to registered nurses and other clinical staff, and perform an important role as part of a multi-disciplinary team.
"But it is a very different role to a registered nurse and cannot and should not be used as a substitute.
"This is not fair on healthcare assistants, who may feel under pressure to perform tasks they are not trained for, and it is not fair on patients, who will rightly be concerned if healthcare assistants are replacing registered nurses.
"This trend highlights yet again that across England we are desperately short of registered nurses.”