Ministers have faced renewed calls for minimum staffing levels in hospitals after an NHS review found staff were too busy to deliver compassionate care to patients.
Dr Peter Carter, general secretary of the Royal College of Nursing, said:
There's an undeniable link between nurse staffing levels and patient mortality and we can't keep failing to address this issue.
Only with the right numbers of nurses, with the right skills, can we ensure patients are looked after with dignity and compassion.
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The study by the National Childbirth Trust and National Federation of Women's Institutes found a lack of staff left some women frightened.
Extracts released by 10 Downing Street suggest Mrs May will say the UK will not settle for a "half-in, half-out" policy.
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