The NHS will experience "pockets of meltdown" this winter as the service comes under increasing pressure, a leading doctor has warned.
Dr Mark Holland, president of the Society for Acute Medicine, said the resilience of medical units was being "put to the test like never before".
It comes a day after the chief executive of NHS Providers, Chris Hopson, said the Government would fail to achieve its aim of a full seven-day NHS without more cash.
He warned that hospitals are cutting services and the NHS risked "slowly deteriorating" as it did in the 1990s.
Dr Holland echoed the warnings of a service under pressure, saying those parts of the NHS where performance was already weak would find the winter months particularly difficult.
He also pointed to the "national emergency" of medically fit patients not being discharged home from hospital. This is partly due to inadequate social care in the community.
He said the current concern was "that hospitals where performance is already weak will find it very difficult to cope during the winter and that will lead to pockets of meltdown".
He added: "At the core of the issues facing the NHS is the rapidly increasing number of delayed discharges. The figures are spiralling and are fast becoming disastrous.
"If hospitals cannot discharge patients then the system comes to a halt.
"We need an overhaul of the discharge and social care process nationally so we can release pressure on front of house services in our hospitals - it is a national emergency."
Delayed discharges - often called bed-blocking - refer to the number of people stuck in hospital who are medically fit to leave.
Figures for July show the equivalent of 184,188 days were lost due to delayed discharges, the highest on record, with estimates suggesting the figure across a year could be as high as 2.7 million bed days lost.
Dr Holland was speaking ahead of the Society for Acute Medicine's conference in Edinburgh. Acute medical units deal with the immediate treatment of adults with a variety of medical conditions who present to hospital as emergencies.
The society published a new report on the performance of acute medical units based on speed of assessment, access to clinical decision-makers with four hours and a consultant review within 14 hours.
The audit, which covered 94 units and 4,140 patients, showed 81% of patients were seen by a consultant within the target, 69% by a competent decision maker within four hours and 69% had an early warning score recorded on arrival. However, only 41% of patients received all three.
Dr Holland said: "The findings are that, while we are still doing well on average and some units are doing very well, there is a spread and variance in what we achieve and, overall, performance has dipped over the last three years.
"This tells us that, despite the brilliant work of acute medicine practitioners to maintain quality and safety - and there are some fantastic examples of staff going above and beyond - our resilience is being put to the test like never before and that is not sustainable in the long-term."
He said it was "vital" that NHS trusts that were struggling to cope were not "hidden" by better performers who average out statistics.
MPs on the Commons health committee will hear about winter preparations later on Monday.
Professor Keith Willett, director for acute care at NHS England, and Pauline Philip, urgent and emergency care director, will be among those appearing before MPs in Westminster.