Patients are less likely to die in hospitals where there are more nurses, researchers have suggested as they warned managers not to replace them with less qualified staff.
Two new studies found that the number of nurses at NHS bedsides in England appears to have an impact on whether patients survive.
One of the studies also warned against shoring up nurse numbers with unqualified or less qualified healthcare support workers.
Published in BMJ Open, experts found lower death rates when there were more nurses working. Meanwhile, hospitals with healthcare support workers in higher numbers had higher patient death rates.
The team looked at 137 acute hospital trusts in England and found 7% higher death rates in trusts where there were higher levels of support workers.
In a subset of 31 trusts (41 acute hospitals), where the team knew the exact staffing ratio per patient, they found that trusts with an average of six patients or fewer per registered nurse had 20% lower death rates compared with trusts with more than 10 patients per nurse.
There was also a slight increase in patient deaths for trusts with the most healthcare support workers.
Jane Ball, principal research fellow at the University of Southampton, said: "At best, healthcare support workers make no difference, but at worst a higher level of support workers is linked to an increased risk of death during a hospital stay.
"There's no evidence of it ever being a good thing (in terms of patient mortality)."
She added that in her view there needed to be "clear guidance on levels of staffing that you shouldn't go below".
In the research, the authors added: "Current plans for workforce development in England and other countries point toward a significant increase in the numbers and proportion of registered support workers and assistant practitioners relative to the number of registered nurses, and registered nurse recruitment remains problematic.
"However, such a shift seems to be at odds with evidence that points toward a more highly trained nursing workforce being associated with fewer adverse events."
Janet Davies, chief executive of the Royal College of Nursing (RCN), said: "The evidence is a clear warning about the impact on patient care and outcomes if we are to have too few registered nurses or are substituting them for healthcare support workers.
"Healthcare support workers are highly valuable staff but they need to complement the registered nursing workforce - not replace it.
"Health services need to work towards achieving the best overall mix of skills - it could make the ultimate difference for patients."
A further study found that a drop in the number of nurses caring for poorly babies is leading to higher death rates.
Published in the Archives of Disease in Childhood journal, experts found that one-to-one nursing for very sick babies in neonatal intensive care dropped by a third - from an average of 9% intensive care days to an average of around 6% - between 2008 and 2012.
Calculations showed that a 10% drop in the proportion of intensive care days on which one-to-one nursing care was provided was linked with a monthly increase in baby death rates of of 0.6 per 100 infants (0.6%) in intensive care.
The British Association of Perinatal Medicine (BAPM) recommends one-to-one nursing care for newborns in neonatal intensive care.
The authors stressed the study was observational and did not prove that low nursing ratios were to blame. But they said it suggested decreases in intensive care nursing "increase the in-hospital mortality rate."
Caroline Davey, the chief executive of premature and sick baby charity Bliss, said: "The Government and NHS decision makers have so far failed to take the necessary action to address these staffing shortfalls, which we have known about for many years."
The Department of Health forced the National Institute for Health and Care Excellence (Nice) to stop its work into safe staffing levels last summer - although some of their recommendations on staff-patient ratios have been leaked. The work has now shifted to NHS Improvement.
A Department of Health spokesman said: "We expect all parts of the NHS to make sure they have the right staff, in the right place, at the right time to provide safe care and there are already more than 10,600 additional nurses on our wards since May 2010, as well as over 50,000 nurses currently in training.
"Our changes to student nursing, midwifery and allied health professionals funding will also create up to 10,000 more training places by the end of this parliament."