Hospitals are struggling to cope as the number of inpatient beds continues to be slashed across the health service, an influential think-tank has warned.
A report from the King's Fund found that the health service in England is facing increasing pressure as it goes into the winter months.
At the same time, blueprints drawn up locally but demanded by NHS England - known as sustainability and transformation plans (STPs) - have set out proposals to cut hospital beds even further.
Last winter, hospitals across the UK declared major alerts and closed their A&E doors, with an average of more than 90% beds full across the service.
At various points, hospitals had around 96% of their beds full and regularly went above 95%. Anything above 85% increases the risk to patients from infection and is considered unsafe.
The new study says that with hospitals already full to capacity, STP proposals in some areas to cut beds are "undesirable and unachievable".
The number of NHS hospital beds in England has more than halved over the last 30 years, from around 299,000 to 142,000.
The report said this is partly because more patients with mental illness and learning disabilities are cared for in the community and patients generally need to spend less time in hospital than in the past.
But it warns that the NHS "now has fewer acute hospital beds per person than almost any other comparable health system".
And it points to a growing shortage of beds as hospitals struggle to cope with increasing numbers of patients with more complex conditions.
It said: "Today there are signs of a growing shortage of beds. In 2016/17, overnight general and acute bed occupancy averaged 90.3%, and regularly exceeded 95% in winter, well above the level many consider safe.
"In this context, proposals put forward in some sustainability and transformation plans to deliver significant reductions in the number of beds are unrealistic."
The report said cutting beds relies on people being cared for closer to their homes and in the community but current intermediate care - the services to support patients leaving, or at risk of going into, hospital - is only sufficient to meet around half of demand.
Cuts to social care are also having a big impact.
Helen McKenna, senior policy adviser at the King's Fund and one of the report authors, said: "Over the years, medical advances and policies to care for more patients in the community have enabled hospitals to treat more patients using fewer beds.
"This has improved care for patients and resulted in significant reductions in the number of hospital beds. There are opportunities to make better use of existing beds and initiatives to capitalise on these should continue.
"But with many hospitals already stretched to breaking point, reductions on the scale we know have been proposed in some areas are neither desirable not achievable. We welcome new requirements introduced by NHS England that local areas must meet before significant numbers of beds are closed."
An NHS England spokeswoman said: "Hospitals have said that they are planning to open up more than 3,000 extra beds this winter and, in addition, work is under way to free up to 3,000 more by improving the availability of community health and social care.
"We have also introduced an explicit test to prevent inappropriate bed closures while recognising that the right number of beds in any geography will partly depend on the age and number of patients in that area, the availability of alternative community services and also the level of the NHS budget that parliament requires local areas to operate within."