A major new study says external factors, such as weather, have little to do with the pressures experienced by A&E departments.
The Nuffield Trust report warns of growing problems of "crowding" in departments when surges of patients overwhelm the system.
Researchers found an 8% increase in cases of crowding since 2010 - compared with just 3% increase in attendances.
Researchers also identified groups of patients facing the longest waits in A&E. These include the elderly, those with long-term conditions and those with more than one illness.
Successful GP practices played a part in ensuring patients did not need to resort to hospital, researchers found.
Ian Blunt, from the Nuffield Trust, said much depended on the review of emergency care under way by NHS England.
He said: “This report shows that many of the external factors suggested to explain the decline in performance on the four-hour target since 2012 only had a small effect. We may have reached the limit of what we can deliver with our current A&E capacity.
"Without either new money to invest in A&E, or new ways to divert people to other parts of the NHS, rising waits may be inevitable."
Dr Jennifer Dixon, of the Health Foundation, said: "As this report shows, A&E departments are completely stretched, aggravated by increasing demand for emergency care. But the solutions are more complex than simply ‘more investment needed’, even if that were forthcoming."
The British Medical Association said the NHS was now "starting to buckle."
Chair Dr Mark Porter said: "Ultimately, politicians have to face up reality: we have an ageing population with more complex medical needs. Demand on the NHS is rising and will only continue to rise. Staff and services are stretched to capacity, workloads are becoming unmanageable and patient care is being compromised.
"The message is stark and clear - without urgent investment in front-line services and real action to put the health service on a sustainable financial footing for the future, the NHS simply will not be able to cope."
Some 1,600 people with back-ache took part in a randomised trial of the pill in Australia.
This meant some people were given fake pills while others received paracetamol.
According to the findings, published in The Lancet, the drug proved to have no impact on speeding recovery, reducing pain or improving quality of life.
In fact, in the study, patients taking placebo pills recovered faster, getting better on average in 16 days rather than the 17 endured by those taking true paracetamol.
Researcher Dr Christopher Williams, from the University of Sydney, Australia, said today: "Simple analgesics such as paracetamol might not be of primary importance in the management of acute lower back pain.
“The results suggest we need to reconsider the universal recommendation to provide paracetamol as a first-line treatment for low-back pain, although understanding why paracetamol works for other pain states but not low-back pain would help direct future treatments."
Efficacy of paracetamol for acute low-back pain: a double-blind, randomised controlled trial.Lancet 24 July 2014 [abstract]
Auditors reported "significant weaknesses" in the management of ten hospital groups including University Hospital of North Staffordshire - which is due to take over the troubled Mid-Staffordshire NHS Trust.
Another 27 trusts had their accounts qualified by the Audit Commission because of specific weaknesses.
The commission went on to notify the Department of Health about 20 NHS trusts because of their inability to balance their books, including the North Staffordshire Trust. Others named included Leeds Teaching Hospitals NHS Trust.
One of the 19 trusts, George Eliot, Warwickshire, was removed from special measures by the NHS Trust Development Authority earlier this month.
The commission is responsible for auditing some 98 NHS Trusts, together with more than 200 clinical commissioning groups. These are the trusts that have not become foundation trusts.
Marcine Waterman, from the Commission, said: “This year auditors are reporting concerns about the financial resilience of a third of NHS trusts compared with a quarter last year.
"This level of reporting is worrying and reflects the increasing risks to the financial sustainability of individual NHS trusts, as they continue to face sizeable financial pressures due to a rising demand for services and the necessary focus on quality of care, whilst balancing the need for continued cost savings.”
The procedures allow couples to have children free of mitochondrial DNA disease.
The British government has backed the move - but regulations will have to be ratified by the UK Parliament.
The procedure, which is being pioneered in Newcastle, UK, involves replacing the mitochondria in human embryos with genetic material from a donor. Mitochondrial DNA provides the fuel for cells and is normally passed down through the female line.
Under proposed rules, children born through the procedure will never be allowed to know the identify of the donor.
The Human Fertilisation and Embryology Authority will have to be satisfied of a significant risk that the child would develop a serious illness and that there is risk of mitochondrial abnormality if the procedure is not used.
Dr Andrew Greenfield, an HFEA scientific adviser, said: "In three years study the expert panel has seen no evidence which suggests that these new mitochondrial replacement therapies are unsafe.
"The scientific direction of travel is clear; and although we have recommended further experiments before treatment should be offered we understand that good progress on these is being made and we expect them to support the conclusions we have reached to date.”
Professor Doug Turnbull, of Newcastle University, said: “We would welcome a firm timetable for this to be tabled for debate in Parliament and become a legal treatment. We have patients waiting who could greatly benefit from this new IVF technique.”