Hospitals have been forced to turn patients away due to immense pressure, with some issuing "black alerts" and cancelling operations and clinics.
Figures released by NHS England show patients have been turned away from busy A&E departments, while NHS trusts told the Press Association they were struggling to cope with patient demand.
Just days before thousands of junior doctors across England go on strike, the NHS has been faced with a surge in people seeking treatment, prompting hospitals to take to Twitter to tell patients not to go to A&E unless it is "life-threatening".
Some hospitals are on "black alert" - the highest level, which means they cannot cope with the number of patients - while others are on "red alert", which means they are under "extreme pressure".
Among those struggling are hospitals in Essex, with Southend University Hospital NHS Foundation Trust and Basildon and Thurrock University Hospitals NHS Foundation Trust both on black alert.
Royal Cornwall Hospitals NHS Trust said it was on black alert, while the health system across Bristol, North Somerset and South Gloucestershire remains on black alert.
Colchester Hospital University NHS Foundation Trust has been on black alert since Tuesday but this was downgraded to "amber" on Friday morning. Other NHS trusts are on red alert.
Burton's Queen's Hospital closed its doors temporarily on Monday to patients arriving in ambulances.
As well as England, Scotland and Wales have also experienced issues with high attendances at A&E.
Analysis of new NHS England figures by the Guardian showed that hospitals have had to send patients elsewhere for treatment 89 times over the last five weeks.
In addition, hospital trusts in England have run into "operational problems" no fewer than 141 times.
A spokesman for NHS England said the figures on patients being sent elsewhere are just under the figure for the same five weeks last year.
He said this winter had so far also been much quieter for "operational problems".
Labour's shadow health minister, Justin Madders, said: "Hospitals are dangerously full, patients are waiting hours in A&E, and some hospital bosses have had no choice but to close their doors in order to cope.
"Ministers have been warned on numerous occasions that cuts to older people's care in the home will have a knock-on effect on the NHS."
Unison union general secretary Dave Prentis said: "It is outrageous that vulnerable patients are being turned away when they urgently need to get medical attention.
"We have constantly warned that the NHS was hugely stretched - those working in A&Es have spent much of the past five years living under the constant threat of services being cut, closed or reorganised."
Professor Sir Bruce Keogh, NHS England's national medical director, said: "I am pleased to say that the NHS had a good Christmas period.
"There were fewer serious operational problems than last year, lower hospital bed occupancy rates and lower levels of illness than usually seen at this time of year."
He said many patients had listened to calls to keep A&E for real emergencies.
He added: "As expected in the first week of January, pressures have emerged in some areas but these are being effectively managed locally. We continue to monitor the situation very closely, and will support hospitals where required."