A&E departments may be understaffed as much as half of the time, according to leaked documents.
Guidance from the National Institute for Health and Care Excellence (Nice) into safe staffing levels in A&E - which was never published following intervention by Government officials - says departments can be overstretched.
It sets minimum ratios for nurses to patients, and points out how hospitals should not draft in staff from other parts of the hospital to fill gaps, due to their lack of expertise in A&E.
It also says there must be a "red flag" warning system for when departments are getting too full and are struggling to cope.
The 58-page document leaked to the Health Service Journal (HSJ) - titled Safe staffing for nursing in A&E departments: NICE safe staffing guideline - follows on from draft guidance published in January 2015 by Nice.
The latest guidance was never published after the Department of Health forced Nice to stop its work into safe staffing last summer.
A 28-page appendix accompanying the guidance, where experts discuss the need for a new calculation to work out staffing ratios rather than using a "historical average" of patient attendance figures, has one section called "issues with current practice".
It says the historical average method is "not adequate to meet demands".
It adds: "Specifically, staffing levels may not meet demand approximately almost half of the time."
The guidance also sets minimum safe staffing levels. It says there should be two nurses to one patient suffering major trauma and two nurses for every one patient suffering a cardiac arrest.
There should also be one nurse for every cubicle in A&E triage, and one for every two patients in the resuscitation area.
For priority ambulance calls, there should be one nurse for every patient.
The guidance also calls for a "red flag" system to be introduced as a "warning sign that something may be wrong with nursing staff levels".
Examples may include delays of more than 30 minutes in providing pain relief, delays of more than 10 minutes in meeting patients' toileting needs and delays of more than 30 minutes in meeting patients' hydration or nutrition needs.
Patients suffering falls, a crowded A&E department, patients going missing and patients leaving the A&E department against advice would also constitute "red flags".
The document also said data should be recorded on staff having to miss breaks or working extra hours (paid and unpaid).
Ministers are known to be against the setting of staff-patient ratios in the NHS, despite research showing that low staffing contributes to excess deaths and poorer outcomes for patients.
The responsibility for safe staffing work has been transferred from Nice to another organisation, NHS Improvement.
An email exchange last summer between Nice's chief executive Sir Andrew Dillon and Health Secretary Jeremy Hunt's office led to Nice ditching its plans to make public its research on safe staffing levels.
Released under freedom of information laws, the email from Mr Hunt's private secretary Kristen McLeod said "it could be confusing for staff and the public if Nice were to release one piece of work on safe staffing now without any context and in isolation of any final guidance on safe staffing levels".
A spokeswoman for Nice said of the latest leak: "Nice has not published guidance on safe staffing in accident and emergency departments and doesn't intend to do so."
The decision to suspend Nice's safe staffing work was criticised by high-profile figures including Sir Robert Francis QC, who led the public inquiry into failings at the Mid Staffordshire NHS Foundation Trust.
Labour's shadow health minister, Justin Madders, said of the leak: "Safe staffing levels are essential for patient safety and were a key recommendation of the Francis report.
"The Government's decision to block these guidelines from being published is deeply concerning and raises a lot of questions about the ability of experts to give independent advice to the NHS.
"Documents being leaked in this way goes to the heart of whether the Tories can be trusted to run the NHS. How can they expect to have the confidence of patients, the public and staff if they can't be honest with them?"
Julie Bailey, founder of Cure the NHS, whose mother was a victim of the problems at Mid Staffs, said: "As a group we remain shocked and concerned that the Government and NHS England suspended Nice's work on safe nurse staffing, a key recommendation of the Francis report that we fought so hard for.
"Safe staffing and nurse-to-patient ratios are essential to ensure patients do not suffer as our relatives did at Mid Staffordshire."
Donna Kinnair, director of nursing, policy and practice at the Royal College of Nursing (RCN), said: "These guidelines were put together by experts, looking at strong evidence, who found a very clear relationship between the number of registered nurses and patient care.
"The evidence for the importance of having the right number of nurses, and the right ratio of nurses to health care assistants, would have led to new recommendations and guidance on the safe range of nurse staffing levels.
"These recommendations would have exposed shortages, and this would have had financial consequences. It is concerning that these consequences may have been a factor in the decision to scrap this important work."