Figures obtained by HSJ reveal a large difference in staff skill mix between different hospital trusts.
The NHS Information Centre figures show some acute trusts employ nearly double the proportion of unregistered clinical support workers as others.
There is also a three-fold difference between hospital trusts in the proportion of nursing and clinical support staff who are in senior roles at band seven or above on the Agenda for Change pay framework.
In many cases the variations are likely to be due to local factors such as the type of services, the way they are provided and the jobs market. For example, it has been suggested to HSJ that some trusts, such as those in areas with recruitment difficulties, may be more likely to place nurses at a higher band in order to make jobs more attractive.
But the Royal College of Nursing said in some cases the figures on skill mix revealed potentially dangerous staffing and supported the case for compulsory regulation of healthcare assistants.
The Lords is due to debate Health Bill amendments on HCA regulation and enforcement of minimum nursing levels in coming weeks.
Royal College of Nursing policy director Howard Catton said: “For large hospitals these variations mean a significant difference in the number of nurses.
“They appear to be greater than would be accounted for by purely clinical reasons, or because trusts are teaching or specialists.
“The reasons may be historical or regional trends or financial pressure.”
Mr Catton said there was a “sweet spot” in nursing levels and skill mix which was most appropriate both for care quality and cost effectiveness, although this would vary between different types of services. He called for the government to investigate what the appropriate ratios would be and issue a mandate.
Some senior nursing figures are hopeful that, assuming the government maintains its opposition to statutory registration of HCAs, it might increase support for other forms of regulation.
For example, Sheffield University Hospitals Foundation Trust nurse director Hilary Chapman said she supported the development of common education and training standards for HCAs among employers.
The figures are for full time equivalents at acute trusts only. They include all nursing staff excluding midwives and health visitors; and exclude clinical support staff working with midwives.