Nurses have reported feeling exhausted, sick and dizzy as the hot weather raises temperatures in hospitals.
One nurse was admitted to A&E with dehydration after working three 12-hours shifts in a row during the heatwave, the Royal College of Nursing (RCN) said.
Earlier this week, the organisation warned that patients and relatives were passing out and vomiting, with temperatures on some wards exceeding 30C (86F).
It also said some nurses were not allowed to take water bottles on to wards.
Kim Sunley, national officer at the RCN, said: "Nurses are now becoming patients themselves due to the heat.
"We have heard from one member who ended up in A&E suffering from dehydration, after working 12-hour shifts back to back in temperatures exceeding 30C.
"Others have reported exhaustion, sickness and dizziness.
"This is not acceptable. Good patient care depends on nurses and clinical support assistants being well enough to perform their jobs effectively.
"It is vital employers adapt working practices to the heat. Both patients and nurses must have easy access to water, and all healthcare staff should be able to take regular breaks, preferably somewhere cool."
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