A&E units across the UK are struggling to hit their waiting time target as winter hits, latest figures show.
Demands on the NHS tend to increase during the colder months because of illnesses like flu and norovirus.
But with winter just getting under way, pressures are already reaching record levels.
Extra money is being invested in each nation, but the four-hour A&E waiting time target is still being missed everywhere.
The mounting problems come as the BBC launches its NHS Winter project, which tracks how the health service is performing down to individual hospital trust level.
As health is devolved, data is published differently in the four UK nations.
Hospitals in England, Wales and Northern Ireland are meant to see 95% of patients in four hours.
In England weekly figures are released. The latest, for the first week of December, show 91.8% of patients were seen in four hours - the worst performance since April 2013.
In Wales the data is published monthly with the figures from October showing just 84.7% of patients were seen in time - with one in 20 waiting more than eight hours.
Northern Ireland is performing even worse - just under 80% of patients were seen within four hours in October.
Scotland has a slightly tougher waiting time target - 98% of patients should be seen in four hours. In September under 94% were.
Analysis: Why is performance deteriorating?
The simple answer is that more people are visiting A&E units. This is true for each corner of the UK, but as the data is most up-to-date for England it can be seen clearest there.
Visits to A&E topped 436,000 last week - that is nearly 30,000 more than the same week the year before.
But it is also the nature of the conditions patients are presenting with that makes a difference.
The best indication of this is the numbers that need to be admitted into hospital as an emergency. They are the most complex cases that often take A&E staff the longest to deal with and lead to beds being occupied.
The numbers topped 110,000 last week - that is one of the highest figures ever recorded.
British Medical Association leader Dr Mark Porter said: "Pressure on NHS services is at a critical point and cracks are beginning to appear.
"While the NHS is used to seeing a spike in demand during winter months, this year emergency departments have experienced a spring, summer and autumn crisis as well, leaving no spare capacity in hospitals as we approach winter.
"At the same time, GP surgeries are struggling to cope with unprecedented levels of demand."
Rob Webster, chief executive of the NHS Confederation, which represents health bosses, said he agreed, adding the pressures were "huge" across the whole system.
"While the winter brings its own challenges, our members regularly tell us that it is 'winter all year round' with pressures being experienced regardless of the time of year."
But Sarah Pinto-Duschinsky, director of operations and delivery for NHS England, said: "The NHS is pulling out all the stops, with local hospitals, ambulances, GPs, home health services and local councils all working hard to open extra beds and seven-day services using the extra winter funding."
How the UK is preparing for winter
- In England an extra £700m has been set aside to help the NHS. This is paying for the equivalent of 1,000 extra doctors, 2,000 nurses and 2,000 community staff, including social workers and physios.
- Ministers in Scotland have announced an extra £8.2m for the NHS to increase capacity and an another £10m to improve the way patients are discharged during winter.
- In Wales the NHS has been given an extra £200m for this year. The money is for the whole health service, but ministers say it will help relieve the pressures in the coming months.
- Ministers in Northern Ireland have released an additional £5m for both hospitals and the community, including ambulance liaison officers to speed up handovers between paramedics and A&E teams.