Nearly one in five local hospital services are consistently failing to hit any of their key waiting-time targets, BBC analysis shows.
Twenty-nine hospital trusts and boards out of 157 have not hit a single target for a whole year.
Northern Ireland is struggling the most – all five trusts have failed their key targets for A&E, cancer and routine operations every time in 2017-18.
NHS leaders said the NHS was facing a very difficult winter.
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Dame Donna Kinnair, of the Royal College of Nursing, said the NHS was going into the coming months "on the back foot".
She said research by the RCN showed hospitals were facing a shortage of both beds and staff – with images of "patients waiting on trolleys in corridors" becoming all too common.
The health departments in each nation have said they are committed to improving waiting times. In Scotland and Wales, specific programmes have been launched to speed up progress.
Northern Ireland's Health and Social Care Board conceded "the waiting times experienced by many patients continue to be unacceptable".
The Department of Health and Social Care in England praised "hard-working staff" and said the extra money being provided to the health service in the coming years would put it on a "sustainable footing".
'My mum is suffering'
Carol Johns has been battling to get her mother, Maria King, treatment since the spring.
Ms King, 84, has been experiencing painful and swollen feet, which has left her struggling to walk and has disturbed her sleep.
Her daughter said she had experienced delay after delay in trying to get scans and appointments with doctors.
One appointment had to be cancelled because the hospital said it did not have enough doctors.
She has even tried to get treatment at other hospitals further away from her home in the Midlands and paid for a scan to be done privately.
But her mother is still waiting.
"I am very frustrated. My mother has not got the quality of life she did. She is suffering."
How quickly are patients meant to be seen?
It depends on where you live in the UK.
In each UK nation, patients are meant to be seen within four hours of arrival at A&E – and by seen, the NHS means either admitted into hospital for further treatment or treated and discharged.
For cancer, each nation expects patients to start their treatment within 62 days of an urgent referral, although there are differences in the way each nation measures that.
The biggest variation in the way performance is judged is seen in the waiting times for non-emergency treatments, such as knee and hip replacements.
In England and Scotland, patients are meant to be seen within 18 weeks, while in Wales it is 26 weeks.
Northern Ireland has a different system in that it only measures part of the patient's wait – once they are under the care of a hospital doctor rather than when they are referred by a GP. It means their wait for tests and scans is not counted in the 13-week wait target.
Is this as bad as it has ever been?
Another way of looking at performance is to judge it on a nation-level rather than by local hospital service.
If you do that performance has sunk to its worst level in recent times.
England, Northern Ireland, Scotland and Wales have failed to hit any of the three targets for more than a year.
It is the first time this has happened across the UK since targets started to be introduced more than a decade ago.
The last time any target was met on a national level was in August 2017 in Scotland.
In Wales, none of its three key targets has been hit for at least five years.
Chris Hopson, chief executive of NHS Providers, which represents managers, said it was clear the system was facing "significant strain".
Which hospitals are struggling the most?
In England, 16 hospital trusts out of 131 missed all their monthly targets. They were:
- Bradford Teaching Hospitals
- Taunton and Somerset
- Guy's and St Thomas'
- Northern Lincolnshire & Goole
- Plymouth Hospitals
- The Royal Wolverhampton
- Mid Essex Hospital Services
- Leeds Teaching Hospitals
- University College London Hospitals
- East Kent Hospitals
- Hull and East Yorkshire Hospitals
- United Lincolnshire Hospitals
- Maidstone and Tunbridge Wells
- East and North Hertfordshire
- Worcestershire Acute Hospitals
- Brighton and Sussex University Hospitals
In Northern Ireland, all five trusts have failed to hit a single target in the past 12 months. They were:
- South Eastern
In Scotland, three out of 14 health boards missed all their targets. They were:
- Forth Valley
- Greater Glasgow and Clyde
In Wales, five out of seven health boards missed all their targets. They were:
- Cardiff and Vale
- Cwm Taf
- Aneurin Bevan
- Abertawe Bro Morgannwg
- Betsi Cadwaladr
What do the hospitals say?
A combination of rising demand and the need to prioritise emergency patients has been blamed by the NHS trusts which are struggling the most.
Plymouth Hospitals chief operating officer Kevin Baber said staff were working hard, but were struggling with the "ever increasing demand".
He said headway was being made in some areas.
Maidstone and Tunbridge Wells said it had had to prioritise emergency patients over the past year – and that had meant others were having to wait longer.
And Gwen Nuttall, from the Royal Wolverhampton NHS Trust, said meeting targets "will always be a challenge" in the current climate.
"Our hospitals, along with others regionally and nationally, are incredibly busy," she added.
Additional data journalism by Will Dahlgreen