While NHS trusts locally make the decisions about staffing levels, the RCN said there is a lack of national oversight about the reduction in staff numbers.
The organisation is calling on ministers to prevent NHS trusts from "continuing with this damaging agenda of cuts" that "impact on patient care".
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A 61-year-old woman was taken to hospital with head injuries after the incident in Islington, north London.
The suspect is believed to have acted alone at Noblesville West Middle School, in suburban Indianapolis.