Poor performance in A&E has "become the norm" for some NHS trusts, MPs say.
A new report from the Commons Health Committee warns 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 a widespread inability to move out patients who are medically fit to be discharged.
Evidence submitted to the MPs' inquiry 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 only 87.9% of patients admitted, discharged or transferred within that timeframe in 2015/16.
MPs expressed concern over a continued fall in standards, saying: "The ongoing decline in performance of type 1 emergency departments against the four-hour target should be regarded as a matter of patient safety rather than a failure to meet a bureaucratic objective."
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", MPs said.
They added: "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."
On staffing, MPs heard from the Royal College of Emergency Medicine (RCEM) which said among the 176 type 1 departments in England, there are currently "insufficient consultants in post to provide even one on duty in every department for even 16 hours per day".
If the number of doctors had kept pace with the increasing number of patient admissions in recent years, there would now be 8,074 doctors working in A&E departments rather than 5,300.
Some trusts also told how neighbouring hospitals were refusing to obey a Government cap on how much they could spend on agency staff, which led to them being "outbid" for workers.
One NHS trust said "there is a seller's market for A&E doctors and the lowest bidder can find themselves struggling to have enough doctors to meet the demand in A&E ...Without increasing staffing levels both during winter months and over the long term, safe staffing levels will not be achieved."
The report also detailed problems with the handover of patients at A&E, which means ambulances are unable to get back on the road to attend other patients.
Modelling by the Centre for Urgent and Emergency Care Research reported that in the year 2015/16, the 10 ambulance services in England lost a total of 407,000 hours through handover delays at A&E.
As an example, one English ambulance trust loses approximately 700 hours every week due to this issue.
MPs also pointed to the major problems in social care, saying they believe it is "underfunded". This impacts on hospitals' ability to discharge patients into the community to care homes or their own homes with a package of support.
They also warned that the Government should not rely on GPs to relieve the pressure on A&E departments in winter, including over the festive period.
Dr Sarah Wollaston, Tory MP and chairwoman of the Health Committee, said: "Accident and Emergency departments in England are managing unprecedented levels of demand. The pressures are now continuing year round without the traditional respite over the summer months as departments try to cope with increasing numbers of patients with complex needs."
Dr Mark Porter, chairman of the British Medical Association (BMA), said: "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."
RCEM president Dr Tajek Hassan said: "Whereas traditionally winter would be a busy period, the system would always, to an extent, recover. We are now failing to see recovery with almost year round pressure resulting in delays and overcrowding."
Health Minister Philip Dunne said: "The NHS is better prepared for winter than ever before, with plans in place to help hospitals cope with additional demand - and the NHS is performing well despite the pressure of an ageing population, with nine out of 10 people seen in A&E within four hours.
"Crucially, we are ensuring the money available to local authorities for social care will rise in the coming years - by up to £3.5 billion extra in 2020.
"We are investing £10 billion to fund the NHS's own plan for the future to reduce pressure on hospitals, and will have spent billions on capital projects by 2020."