Hospitals across the UK have issued "black alerts" due to immense pressure, with some turning people away from A&E and others cancelling operations.
As the effects of winter continue to bite - and just days before a strike by thousands of junior doctors - patients are being told to only attend A&E in emergencies.
Some hospitals are on "black alert" - the highest level, which means they cannot cope with the number of patients - while others have taken to Twitter and their own websites to tell people to stay away.
Essex is suffering problems, with Southend University Hospital NHS Foundation Trust and Basildon and Thurrock University Hospitals NHS Foundation Trust both on black alert.
A spokeswoman for the Southend trust said it had "a very high number of patients visiting the emergency department and a higher than usual number of acutely ill patients requiring a hospital bed".
She said some planned operations and outpatient clinics have been cancelled as a result, although cancer-related and emergency surgery will still go ahead.
Sue Hardy, chief executive at Southend University Hospital, added: "The hospital is currently extremely busy dealing both with the high number of patients coming through our doors and the number of very ill patients who need to be admitted to hospital.
"The emergency department is very busy and we have to warn patients that those attending with 'non-urgent' conditions will wait much longer than usual as patients are prioritised according to how unwell they are."
Basildon is still on black alert, meaning there is more demand coming through A&E than beds available in the system.
Tweets from the hospital say: "Please call NHS 111 if you urgently need medical help or advice but it's not a life-threatening situation" and "our A&E is still very busy".
Colchester Hospital University NHS Foundation Trust has been on black alert since Tuesday but this was downgraded to "amber" this morning.
The hospital's A&E experienced an influx of patients just after midnight on Tuesday, meaning there were not enough beds and "the hospital could not cope", a spokesman said.
He added: "While we're clearly in a better place than we were yesterday, there was a big emphasis today on not letting our foot off the gas and on 'maximising discharges'."
The local health system across Bristol, North Somerset and South Gloucestershire has been on 'system-wide black escalation' status all week.
Extra staff have been drafted in to the minor injuries units in Clevedon and Yate and at South Bristol's Urgent Care Centre. The availability of GP out-of-hours appointments has also been increased.
Meanwhile, Maidstone and Tunbridge Wells NHS Trust said it was "currently in a period of high demand for services" and "high volumes of patients through its emergency departments each day".
A statement said some patients have experienced long delays, adding: "On occasion it will be necessary to transfer patients between hospital sites in line with our treatment pathways, and also to make most effective use of all our inpatient facilities."
Torbay Hospital has been on red alert for days.
A hospital spokeswoman said: "Red alert is about being under extreme pressure, with a high demand for beds but not at the stage where we are turning ambulances away.
"We have opened up extra beds on an escalation ward."
On Monday, a South Wales health board asked people to avoid both its A&E units because they were "extremely busy".
Officials at Abertawe Bro Morgannwg University (ABMU) Health Board said the Morriston and Princess of Wales hospitals in Swansea and Bridgend had a high number of emergencies which have caused delays in the system.