Experts said that pressures normally only seen on the health service during the winter months are "increasingly visible at other times of the year."
The new report by health charities the Nuffield Trust and the Health Foundation, found that a number of indicators which show when the NHS is struggling in colder months are increasingly becoming apparent throughout the rest of the year.
Problems which are traditionally confined to winter but are now found in summer include long "trolley waits" - where emergency medics have decided to admit a patient but they are forced to wait for hours before a bed is found - and worsening ambulance response times.
"Problems that were usually confined to the winter months are now increasingly being experienced at other times of the year," the authors wrote.
"The four-hour A&E target has not been met nationally for over two years. Patients waiting to be placed on a ward after a decision to admit (trolley waits) is also an increasing issue, with the number and proportion of patients affected in summer 2015 being at the same levels seen in the winter of 2012/13, and worse than those in the winters of 2010/11 and 2011/12."
They added: "So problems with emergency care performance usually only seen during winter seem to have become 'the new normal' for the rest of the year."
There is usually additional pressure on the NHS in the winter because it is a time where additional illnesses and the colder weather can affect the most vulnerable groups in society.
The experts said that after a difficult summer, the NHS entered the 2015/16 winter from a poor starting point which could lead to a "downward spiral" in performance.
"Every year, the winter months produce a combination of circumstances that understandably make it harder for the NHS to cope," said lead author of the report, Dr Elizabeth Fisher of the Nuffield Trust.
"Respiratory problems, strokes and heart attacks are all more likely to occur in cold weather, for example, while flu and norovirus are also more common then. We see the effect of these pressures every winter when the NHS shows dips in performance against some measures.
"But the new and worrying development we've identified in our analysis is that although those factors aren't present in summer, the summer months have now started to show performance similar to that seen in past winters.
"This has a knock-on effect in the months that follow, particularly on planned operations, thus making it harder for the Health Service to cope each winter. It's then more difficult for hospitals to recover the following summer, which could lead to a continued downward spiral."
A number of key indicators of performance also showed a bleak picture during the winter of 2014/15, including:
* A big increase in the number of trolley waits - which are defined as patients having to wait between four and 12 hours after a decision has been made by an A&E doctor to admit them to the main hospital.
* The number of patients waiting more than 12 hours after a decision was made to admit them to the hospital reached a peak of 270 in the first week of 2015, a significant increase on the same week over the previous four years. In that week for both 2011 and 2012, the number was only seven; in 2013 it was 11 and in 2014 only 33.
* An increasing number of ambulances queuing outside hospitals - the figure stood at around 60,000 each winter until 2014/15 when it increased to 96,150.
Tim Gardner, senior fellow at the Health Foundation, added: "Infection rates, bed capacity, the health of older people and how they are supported in the community and the rising demand all contribute to the pressure the NHS experiences during winter. All these factors need to be understood better, in particular the impact of threadbare social care."
Commenting on the report, Dr Clifford Mann, president of the Royal College of Emergency Medicine, said: "A&Es are under pressure all year round with more attendances in the summer and more admissions in the winter. Recent reductions in the number of available beds exacerbate an already overstretched system and in consequence more and more patients experience unacceptable delays.
"The resources needed to address these challenges are evident to all; sufficient permanent staff, both doctors and nurses in our A&E departments and beds promptly available for patients when needed."
Heidi Alexander, Labour's shadow health secretary, said: "Under this Tory Government, the NHS not only has a winter crisis but a spring, summer and autumn crisis too. Hospital departments have become dangerously full, patients are waiting hours in A&E, and delayed discharges have hit a record high.
"Ministers might be in denial, but the reality is this crisis is a direct consequence of decisions they have made. Tory cuts to social care have left 300,000 fewer older people getting the crucial help they need than when David Cameron became Prime Minister. This is leading to more and more older people turning to A&E because they can't get the right care in the community.
"Ministers can't keep turning a blind eye to this situation. We need urgent action to improve hospital care in the NHS and repair the damage that has been done by this Government's policies."