Patients needing emergency care have faced a "miserable" time, doctors say, as data shows record-long delays in England in the past month.
Hospitals reported huge pressures in December, with one in five A&E patients waiting over four hours.
A key problem seems to have been a shortage of beds on wards.
Nearly 100,000 of the sickest patients were forced to spend over four hours on trolleys and in corridors after their time in A&E as beds could not be found.
Delays were also experienced by those brought in by ambulances.
One in six crews had to queue outside A&E units for more than 30 minutes, waiting to handover patients to hospital staff - the target is 15.
Royal College of Emergency Medicine president Dr Katherine Henderson said: "The NHS is struggling to escape its spiral of decline.
"This will have been a miserable Christmas period for many patients and staff alike."
NHS medical director Prof Stephen Powis said it had been a "very busy" period, compounded by an earlier outbreak of flu than had happened in previous winters.
He said the health service was looking to recruit more staff and open more beds to help relieve pressure in the future.
In the meantime, hospitals have been forced to take emergency steps to relieve pressure.
Many have postponed routine operations to try to free up space.
And some hospitals have even had to introduce emergency protocols and re-route ambulances to other nearby services.
There were more than 120 cases of this happening last month - with some hospitals instigating the measure, known as an A&E divert, multiple times.
'My father-in-law spent 25 hours waiting for a bed'
Mark Newton's 83-year-old father-in-law, George Bufton, was taken to Shrewsbury Hospital in the first week of January.
He had a chest infection and a suspected gall bladder problem - and this came just over a month after a major bowel cancer operation.
He spent 25 hours waiting for a bed to be found.
Mr Newton said he was "gobsmacked" by what he saw.
At one point about 20 trolleys were waiting in the corridor, with queues of ambulances outside A&E.
He said staff were doing an "admirable" job in the circumstances but there was just not enough money or staff to cope.
"It was utter mayhem. This can't continue. Something must be done about it," Mr Newton said.
How bad has it been?
December was certainly the worst month since the four-hour target was introduced, in 2004.
Just 79.8% of patients spent less than four hours in A&E - well below the 95% target.
Three trusts - Norfolk and Norwich, Stockport and Hull - all saw performance drop below 60%.
Meanwhile, a group of Midlands trusts - Shrewsbury and Telford, Leicester, Birmingham and United Lincolnshire - were all among the 10 worst performers.
Waiting times have been getting gradually worse over the autumn - October and November saw record worst levels set as well.
But the problems in emergency care has not just been confined to A&E waits.
The number of patients who then needed to be admitted on to the ward and faced long delays also hit a new worst level.
There were 98,500 patients who faced a so-called "trolley wait" of a further four hours.
That is nearly one in four of the patients who needed to be admitted.
These "trolley waits" can be in corridors or temporary waiting areas, either in A&E or just outside.
Patients who are admitted are often the sickest of those who attend A&E.
The waiting time targets for cancer treatment and routine operations, such as knee and hip replacements, have also been missed.
What is the cause of the pressures?
Hospital bosses acknowledged it had been one of the most difficult months they had seen - and this comes despite the extra money the government has put in this year, which saw the NHS budget grow by 3.5%.
There are a variety of reasons behind the problems.
Prof Joe Harrison, chief executive of Milton Keynes Hospital, said his staff had been seeing some very sick patients.
He said it had mainly been a combination of older patients and children, who seemed to have been particularly hard hit by flu and respiratory problems.
But the figures also show the number of beds closed to contain vomiting and diarrhoea outbreaks rose by more than 60% compared with last year, with 760 beds closed on average each day last month - nearly 1% of the bed supply lost.
Prof Harrison also said the social-care system in the community, which hospitals rely on to discharge patients into, seemed to have been struggling more than it had been last year.
"Keeping people safe has been our main priority. There is significant pressure, not just in this hospital, but across the NHS system as a whole," he said.
Richard Murray, from the King's Fund think tank, said he believed there were too few beds in the system.
"Hospital bed numbers have fallen significantly over the past decade. With hospitals full to capacity, it is clear that bed reductions have gone too far."
What about the rest of the UK?
The difficulties have not just been confined to England - none of the three core targets covering A&E, routine operations or cancer is being met in the other three nations.
In recent days, the Hywel Dda health board, which is in charge of services in west Wales, said it had had to cancel planned surgery at a number of local hospitals because of the "challenging" situation.
The British Medical Association said services across the nation had been stretched, with some reports cancer surgery had had to be cancelled too.
The four-hour performance figures for December will not be released by the Welsh government until later in the month.
Scotland's figures for the festive period are not available yet either - but November was one of the worst months on record.
Meanwhile, in Northern Ireland, which has the worst record when it comes to A&E delays, there is concern continuing strikes by nurses will have exacerbated the winter pressures.
While A&E departments have remained running during the strikes, the second round of which took place this week, there have been reports hospitals have struggled to discharge patients quickly enough.
And this has a knock-on effect of causing delays admitting patients.
According to the latest figures, which date back to September, a third of patients were waiting longer than four hours in A&E, which was twice as bad as the situation in England and Scotland during that month.