A major NHS hospital has been so overwhelmed that it told senior doctors to make “the least unsafe decision” when treating patients.

Medical groups have voiced concern that Norfolk and Norwich hospital trust’s instruction to its consultants this week showed it was struggling so much to cope with the number of people needing care that patient safety was being put at risk.

At the time the hospital had no spare beds, a full accident and emergency department, 35 patients waiting on trolleys to be admitted, and had declared a major internal incident.

In its message, seen by the Guardian, it said: “We would like you to know that the trust will support you in making difficult decisions that may be the least unsafe decision, and we would appreciate your cooperation over the coming days with this.”

The circular from the Norwich hospital added: “We are facing our most challenging situation with our trust today,” because it was so overcrowded and unable to find a bed for the 35 patients doctors had decided needed to be admitted as emergencies.

Dr Julia Patterson, a spokeswoman for EveryDoctor, which campaigns to improve doctors’ working conditions, said the trust’s message meant “optimal care is unavailable now for some NHS patients [because] ‘the least unsafe option’ is the best we can offer”.

“When hospitals are so full that there are no intensive care beds, no hospital beds at all, and essential operations are being cancelled because there’s simply no one to do the surgery, then every option carries undue risk.”

Dr Sue Crossland, the president of the Society for Acute Medicine, said the message was worrying. “We would always support our members to make safe decisions, despite the pressures we face from all sides. We acknowledge the dire situation that we all find ourselves in, but we must always make safe decisions and give our patients the best care possible. Patients and their families deserve to know that trusts are providing the best and safest care.”

Growing numbers of hospitals are coming under such strain that they are having to declare “black alerts”, meaning they need help to cope with their caseload. Last week 10 A&Es were so busy that they temporarily diverted ambulances to another hospital nearby.

Hospital bosses in England warned on Friday that they may not be able to deal with the risks they will face during the winter surge in demand. A lack of staff, the severity of patients’ illnesses and the unavailability of doctors due to the NHS pensions tax dispute have left trusts ill-prepared, according to NHS Providers, which represents all 240 trusts in England.

“With demand continuing to rise at rapid rates, severe workforce shortages, capacity issues and a lack of a sustainable solution to social care, we anticipate winter will be very difficult this year,” said Saffron Cordery, the organisation’s deputy chief executive.

“NHS staff continue to go above and beyond to treat patients safely and as quickly as possible. But there is no denying that NHS trust leaders are the most worried we have ever seen them.”

The NHS has recorded its worst-ever performance against key waiting times for A&E care, cancer treatment and planned operations in each of the last two months.

Pressure has become so great that thousands of patients a week are waiting with ambulance crews for at least half an hour outside A&E because they are too busy to accept them.

Last week 11,785 patients waited 30-60 minutes to get into an A&E and another 4,469 faced a delay of at least an hour, figures from NHS England show. That meant one in six patients waited at least 30 minutes compared with one in 10 in the same week last year.

Hospitals were 95% full last week, 10% higher than the 85% occupancy which doctors say is the safe level.

Cuts mean the NHS is entering winter with the fewest hospital beds available ever. However, many trusts have created extra beds in “escalation areas” to help deal with the surge in demand over winter.

Almost 900 beds a day are being lost because of outbreaks of norovirus, the winter vomiting bug. Flu rates are also higher than last year.

Public Health England figures released on Thursday showed 23 people had died in the UK since October as a result of flu, compared with 11 in the same period last year.

Erika Denton, the trust’s medical director, said: “The hospital was under extreme pressure earlier this week with very high emergency department attendances and a rise in admissions. A hastily worded email was sent to Norfolk and Norwich University Hospital doctors on Tuesday, which had a number of actions to help address the challenges we were facing.

“The trust is resolute in its determination to provide timely, safe, high-quality care to all our patients, and our staff are diligently working to ensure this happens.

“To help, it’s more important than ever for the public to help NHS staff by getting flu jabs, following advice on the NHS website if they have norovirus, using the NHS 111 phone or online service for advice on urgent medical needs, and consulting their local pharmacist for advice on minor ailments.”