The ability of the NHS to cope with winter has become worse over the past three years, according to a new poll of doctors.
A survey of 457 medics for the British Medical Association (BMA) found that 78% believe the pressure on the health service means it is struggling more now than ever.
Winter is usually a tough time for the NHS, with demand for services increasing, particularly around outbreaks of flu and very cold spells.
The festive break also places hospitals under more pressure as community services to support the disabled and elderly may be run with fewer staff.
On top of that, delayed transfers of care - when patients are medically fit to leave hospital but are stuck in beds while their social care package in the community is arranged - are at their highest levels.
Last month, MPs on the Commons Health Committee said poor performance in A&E has "become the norm" for some NHS trusts.
They warned the NHS could face a "substantially more difficult" winter this year than last, with increasing demand for services, trusts suffering due to too few staff and delayed discharges.
Evidence submitted to MPs showed that A&E departments are now routinely missing the national target to deal with 95% of patients within four hours.
Major type 1 A&E departments - those that are located in hospitals - perform the worst, with just 87.9% of patients admitted, discharged or transferred within that time frame in 2015/16.
MPs said that while in the past, NHS trusts would experience their most intense problems in the winter and would enjoy a respite in the summer, now "pressures are high year-round and just reach a more intense peak during the winter".
They said: "The winter of 2015/16 was mild and the flu vaccine worked. We heard of a fear amongst leaders of acute NHS trusts that 2016/17 could be substantially more difficult.
"It is both significant and concerning that, compared to previous years, hospitals are working from a much lower base in terms of their performance as we enter the winter period.
"The decline in performance of emergency departments, which is usually associated with winter pressures, has become the norm for some NHS trusts."
In the new BMA poll, medics said previous winters had already seen them with unmanageable workloads, including in GP surgeries.
Dr Mark Porter, chairman of the BMA's council, said: "These figures are cause for serious concern as, while there is an ever-increasing demand for health services across the NHS, this is hugely exacerbated during the winter months. Demand is now so great that hospitals are experiencing a year-round crisis, with a lack of available beds preventing the system from coping with a seasonal spike in demand.
"Frontline staff are working flat-out but the system can't cope with the number of patients needing to move through acute care, as the entire system is congested.
"It is vital that there is sufficient capacity across the entire health and social care system, including in accident and emergency departments, general practice and social and community care. Doctors are particularly concerned that a lack of investment and resource in social care is increasingly impacting on the provision of healthcare, especially in winter."
Shadow health secretary Jonathan Ashworth blamed a lack of funding for the "crisis" facing A&E departments.
The Labour frontbencher said: "Today's news that the NHS is struggling to cope with the number of patients in our hospitals is another warning to the Government as winter approaches.
"A&E departments are in crisis, around 3.9 million people are on the waiting list and there is a real concern that this situation could deteriorate over the winter months."
He added: "The Tories need to take urgent action to ensure patients get the care they need and to avoid a crisis in our health system this winter."
An NHS England spokesman said: "Winter bugs and illnesses always bring more pressure but this year hospitals began planning earlier, flu vaccination rates are generally encouraging and we are working far better with senior local council officials in health and social care right across the country.
"The General Practice Forward View, also sets out a plan, backed by a multibillion-pound investment, to stabilise and transform general practice and strengthen wider primary care services."