Healthcare assistants (HCAs) are being used as "nurses on the cheap", with some working beyond their expertise, according to a new report.
Despite a lack of training, HCAs are often left unsupervised to plug gaps in NHS care because of nursing shortages, the study from Unison found.
Some are being told to give patients medicines, despite not being qualified to do so, and others are left in charge of several patients that should have one-to-one care.
The union surveyed almost 2,300 HCAs in the UK working in a range of healthcare settings, from GP surgeries to A&E.
It found 45% of HCAs felt comfortable that what they were being asked to do was appropriate to their level of competence, but 18% were sometimes asked to work beyond their competence, with a further 6% saying this happened often.
Two-fifths had not been given training for some aspects of their job, such as carrying out clinical observations, inserting cannulas or catheters, and caring for people with dementia.
Some HCAs said they were expected to step up to more complex work if wards were busy or if doctors did not "trust" agency nurses.
One HCA said: "Sometimes when there is a staff shortage we have to carry out some responsibilities which I'm not sure if they are part of my role. For example, doing admissions and discharges and sometimes looking after two or three patients that require one-to-one watching."
Another said: "Sometimes we are not allowed to do certain things but - if busy - the situation changes and we are." One said: "I do the ward round for my patients with the doctors as they simply do not trust agency nurses."
Many HCAs said they were concerned about giving medicines. One said: "We are asked to give patients tablets by the nurse when the nurse should be staying with the patient to make sure they take their tablets safely.
"Nurses are signing for tablets and later finding that the patient has not taken them. As health carers we are not trained to give medication to patients."
Another said: "Within my job role I have been approached by the nurses on duty to give the patient medication which I always refuse and insist that they put the medication into the patient's mouth and I will give the water to the patient straight after.
"I am very concerned about giving patients medication as I am not a nurse and it is not in my job and mistakes can happen. I would like some clarification into this to know where I stand."
Some HCAs said they were asked to do tasks that only nurses should be seen to be doing, such as replacing bags of saline on a drip or writing care plans.
More than half (55%) felt staffing levels were inadequate on their wards, while others said they were frustrated by the lack of training and development for their role.
Around 60% felt there was inadequate career progression, with many wanting to train as a nurse or other health worker.
According to Unison, there are over 400,000 HCAs and clinical support workers in the UK, typically delivering around 60% of patient care.
Yet, the Unison report found many are struggling to make ends meet financially, and several described their job as "dogsbody", "Jack of all trades", "skivvy" or "cheap nurse".
Unison deputy head of health Sara Gorton said: "Healthcare assistants are undervalued, increasingly overworked and not getting the support they need at work.
"Their responsibilities have increased massively - from feeding patients to now carrying out skilled medical procedures.
"They are essentially doing jobs previously done by nurses yet this is neither reflected in their pay nor in their career opportunities, so they're struggling to make ends meet.
"Many could earn more stacking supermarket shelves than they can looking after patients. It's nursing on the cheap and patients ultimately suffer as a result."
A Department of Health spokesman said: "Healthcare assistants are a vital part of hospitals and must be supported to deliver safe, high quality patient care - that's why there are 11,300 more nurses on our wards since May 2010 and an extra 50,000 in training."