More than 900 nurses were polled
Hospital patients are being treated in mop cupboards, storerooms and kitchens because wards are full, a survey of nurses has suggested.
Nearly two-thirds of the 900 nurses who took part in the poll said they were aware of cases of patients treated in areas not designed for care.
Some told the Nursing Times magazine that this puts patients at risk.
The government said local managers should ensure services were coping, but said overall care was good.
But while the practices cannot be interpreted as routine on every ward, the fact it is happening has raised alarm within the nurses' union.
Janet Davies, of the Royal College of Nursing, said: "While there may be, in occasional instances, legitimate reasons for treating someone in a non-clinical area, we are concerned at these findings.
"As pressures inside the NHS start to rise due to budget cutbacks, it is vital that nurses and other healthcare staff are confident that their concerns over failures in patient care will be heard."
The nurses highlighted the lack of access to equipment and infection control as the main concerns to patient safety.
One nurse who responded to the poll said: "If a patient's condition suddenly deteriorated resulting in them having a cardiac arrest we would not be able to get the crash trolley to them."
The survey revealed that some nurses had been asked to treat extra patients in wards that were already full. Some also said fire exits had been blocked.
The survey was carried out by asking nurses across the UK to fill in an online questionnaire.
A spokesman for the Department of Health said local managers should be making sure services were coping and she urged nurses with concerns to come forward.
She added: "The vast majority of NHS patients experience good quality, safe and effective care and that we are one of the world leaders in the international drive to improve the safety of healthcare.
"However, we acknowledge there is more to do and will continue to strive to make services even safer."
But Katherine Murphy, of the Patients Association, said: "This is completely unacceptable. Yet again, this survey highlights the gap between rhetoric and reality in the NHS lottery of care."
Liberal Democrat health spokesman Norman Lamb said: "It is absolutely unacceptable that patient care is being compromised in this way."
And shadow health minister Anne Milton said: "This is truly shocking."