0600 GMT January 20, 2019
The advice comes as Milton Keynes University Hospital is telling people only to attend for emergency treatment, BBC reported.
Two ambulance trusts in England, east and northeast, say they are on the highest alert.
Separately, a doctor apologized on Twitter for ‘third world conditions’ in the hospital where he works in Stoke.
The delays are likely to affect tens of thousands of operations and outpatient appointments.
NHS England's National Emergency Pressures Panel, which met for the second time on Tuesday, said that it had extended the deadline for deferral of all non-urgent inpatient elective care — such as hip or knee replacements — to January 31, to free up capacity for the sickest of patients.
But it said that cancer operations and time-critical procedures should go ahead as planned.
Each year, the service comes under increased pressure in the winter, largely as a result of an increase in certain illnesses over this period, such as flu.
It is too early to judge how bad this year's pressures are compared with previous years.
The NHS does release regular statistics, but these are not real-time — meaning there is always a lag of a week or two to learn the full picture.
Milton Keynes University Hospital said that it was facing pressure as a result of ‘very high numbers of patients seeking emergency medical care’.
It said that the hospital had opened an ‘unprecedented number of escalation [extra] beds’ in order to provide care for acutely unwell patients and warned that patients attending its emergency department would be treated in order of clinical priority.
The hospital's chief executive, Professor Joe Harrison, said, "We are seeing very high numbers of very sick patients in the emergency department and fewer patients being able to be discharged — many because they also remain in need of acute care.
"I would urge people to avoid the hospital's emergency department unless it is a genuine emergency.
"Staff are working under incredible pressure to look after the patients in our care. I cannot overstate that. We are doing our best in extraordinarily difficult circumstances."
The NHS is always under great pressure in the New Year — with some patients holding off over the festive season before going into A and E.
Hospitals have planned intensively for the higher anticipated demand over winter.
But the service seems under even more strain than previous years.
The fact that NHS England has felt the need to call for further restrictions on non-urgent surgery and outpatient appointments underlines concerns at the highest level.
NHS England notes a higher number of flu cases and can only hope that its own warnings about a flu outbreak don't materialize.
'Severe and sustained pressure'
Meanwhile, Dr. Richard Fawcett, whose Twitter profile lists him as a consultant at University Hospitals of North Midlands NHS Trust, apologized via Twitter for ‘third world conditions’ in his hospital department.
The hospital trust responded by saying it had been under ‘severe and sustained pressure over the Christmas period’ and this had ‘continued into the new year’.
The trust's medical director, Dr. John Oxtoby, added, "Our staff want the very best for our patients and at times they find the situation frustrating, which can be reflected on social media.
"However, we are a leading trauma, stroke and cardiac center and have been regularly praised by external independent commentators for the quality of compassionate care provided at our hospitals despite all our pressures."
Ambulance services in the north-east and east of England said that they had reached their highest operational alert level — number four.
The North East Ambulance Service said that it had been hit by ‘unprecedented levels of demand’ over the Christmas and New Year period.
It said it was "asking some patients, where appropriate, if they have alternative transport options, such as a family member".
Between December 23 and January 1, it took more than 40,000 NHS 111 calls and more than 16,000 999 calls — compared to around 28,000 NHS 111 calls and 15,000 999 calls in the same period last year.
It advised members of the public to consult a GP, pharmacist, and local walk-in center or to call NHS 111 if they did not require emergency care.
A spokesman for the Scottish Ambulance Service said it had been at level three since Friday.
A spokesman added, "We are experiencing a high call demand, but that is not unusual at this time of year."
A spokesman for the Northern Ireland Ambulance Service said Northern Ireland used a different system.
There are four major hospitals served by the ambulance service and GP and 999 calls are being redirected away from the Ulster Hospital in Dundonald, County Down to the other three major hospitals, which are currently operating normally.