More than 20 hospitals in England have had to declare a black alert this week after becoming so overcrowded that they could no longer guarantee patient safety and provide their full range of normal services.

Unprecedented numbers of patients requiring care has led to at least 23 hospital trusts declaring they cannot cope since Monday, inquiries by the Guardian have established.

Struggling hospitals have been forced to take highly unusual steps in order to manage a surge in demand for care. They include cancelling cancer operations, treating adults in children’s wards and even closing a birthing centre to help cope with a sudden influx of patients who need to be admitted for treatment.

Hospitals are expected to come under even greater strain over the next few days as unusually cold weather sweeps across much of the country, which doctors fear could prompt a spike in serious breathing problems, among other conditions.

University hospitals of Leicester NHS trust was under so much strain on Tuesday that it declared a “system critical incident” – even higher than a black alert – and had to make patients wait in ambulances before being offloaded into the care of A&E staff.

The Royal Surrey hospital in Guildford, which is used by many of health secretary Jeremy Hunt’s constituents, declared black escalation status on Monday because it had become “extremely challenged” by the sheer number of patients whom it needed to find a bed for. At one point that day it had 27 patients who needed a bed but nowhere to put them.

Lewisham hospital in south London became so full on Monday it had run out of beds and in an email to staff it described the situations as “critical” and “not safe”. The hospital had nowhere to put the next patient brought in as an emergency by ambulance, it added.

Under guidance which NHS England issued to NHS acute trusts last October ahead of this winter, hospitals were told to declare what is called a black alert when they have become “unable to deliver comprehensive care [and] there is increased potential for patient care and safety to be compromised”.

The NHS document outlining the operational pressures escalation levels (Opel) system explains that Opel 4 means that “decisive action must be taken ... to recover capacity and ensure patient safety” in the face of “rising system pressure”. That black alert is what most of the 23 trusts have gone onto this week, though some have described it as a “serious internal incident” or in similar terms.

The Royal College of Emergency Medicine, which represents A&E doctors, warned that the already chaotic situation in the NHS could get even worse as the winter weather worsens and that the A&E system is at risk of breaking.

“Emergency departments are overflowing with patients, internal major incidents are being declared around the country and staff in emergency departments are struggling to cope with the immense demand being placed on their services,” said Dr Taj Hassan, president of the Royal College of Emergency medicine.

“These crowded environments are stretching the clinical workforce to their limits and, more importantly, at times are unsafe for patients.”

He urged ministers and NHS leaders to draw up an urgent action plan to rescue A&E care to avoid the quality of care patients receive starting to deteriorate. “Without dedicated funding and planning, both patients and staff will increasingly suffer”, he added. However, it is understood that reports of a package of emergency funding for A&E units are wide of the mark and that the Department of Health will not be producing any more money, despite mounting problems.

On Tuesday the Royal United hospital in Bath closed its Paulton Birth Centre for two weeks, which is usually used by women having a low-risk pregnancy who want a natural birth, and began using it to house medical patients instead.

In an email to staff it explained: “This is because we are experiencing unprecedented bed pressure across the health community and the current bed spaces in Paulton are required to take care of medical patients in a safe environment.” Mothers-to-be left disappointed by the temporary closure could have their babies at an unspecified location elsewhere, it added.

Two days in a row Broomfield hospital, Essex, had to tell a patient with oesophageal cancer that they could not have their scheduled oesophagectomy – which could cure them – because it had no free intensive care bed in which to care for them after the procedure. One cancer specialist at the trust said: “I have never seen so many cancer operations cancelled.”

A spokesman for the Leicester trust, which runs the city’s two main hospitals, said: “The system critical incident, the top level alert, was put in place [on Tuesday] at 8am and we finished it at 1pm [on Wednesday]. This is an alert higher than Opel 4. It’s essentially when this starts to impact part of your business or other businesses.

“For us, [Tuesday’s] situation impacted on the wider health system. We had to hold ambulances at our A&E as they couldn’t offload patients. We were at system critical; that’s the highest point.”

Its main problem was its inability to discharge older patients, in order to free up beds, because local social care was inadequate, the trust added. It had also put more staff on to its frontline and cancelled elective surgery to help it cope.

Caroline Landon, chief operating officer at the Epsom and St Helier trust in south London, told the Guardian: “For a short period last week we used our paediatric day case units to care for adult patients who urgently needed a hospital bed. We also used our adult day case unit and medical infusion suite for the same purpose.”

A doctor at the Maidstone and Tunbridge Wells NHS trust said it had decided not to declare an internal emergency because its bosses did not want bad publicity. Things are so stretched there that she had seen a patient with urinary problems being given a bed bath in the middle of a corridor, with just a plastic sheet to shield her from the rest of the ward.