Royal College of Nursing (RCN) director Janet Davies told the BBC the findings were "very concerning but not surprising - we have been hearing about this for some time from nurses".
She added: "What we are finding is the pressures, particularly in hospitals and community areas, are getting greater as we have got more patients who are sicker coming through the system.
"We are seeing nursing posts frozen or cut in order to try and balance the books, it's obviously something we have been looking at for some time."
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Mladic, now 75, was the military commander of Bosnian Serb forces in the 1990s, who fought against the Bosnian Croat and Bosniak armies.
Weather: Heavy rain and strong winds in the north
His representatives said Cassidy, who was hospitalised last week, died "surrounded by those he loved."