Having hospital nurses care for more than eight patients each during the day creates an "increased risk of harm", an NHS watchdog has warned.
The National Institute for Health and Care Excellence (Nice) has drawn up draft guidelines to address levels of nursing staff following the Francis Inquiry into the catastrophic care failings at Mid Staffordshire NHS Trust.
The watchdog said there is more risk of harm if there is a lower ratio of nurses to patients, but stopped short of stipulating one to eight was an absolute minimum, stating flexibility was required on a day-to-day basis.
Professor Gillian Leng, deputy chief executive and director of health and social care at Nice, said: "There is no floor or ceiling number on the required number of nursing staff that can be applied across the whole of the NHS.
"What the safe staffing advisory committee concluded was that assessing patient needs was paramount when making decisions about the number of nursing staff and planning should allow for flexibility on a day-to-day or shift-by-shift basis.
"This guideline is an important step forward in addressing the issues of having adequate numbers of nursing staff within the NHS."
The guidance for adult inpatient wards in acute hospitals in England said nurses should be aware of "red flag events" which require an immediate response, such as an urgent need for additional nurses.
They include patients not being provided with help to go to the bathroom, not being asked about their pain levels or delays in providing medicines, Nice said.
The draft guidelines also call for hospital boards and senior management to take greater responsibility and include step-by-step guidance on how to determine the number of nursing staff that should be funded.
A Nice spokesman said: "The draft guideline recognises that if each registered nurse is caring for more than eight patients during the daytime on a regular basis, there is an increased risk of harm.
"It stresses the importance of checking if patient needs are adequately being met in these situations.
"The lead nurse should consider any 'red flag events' as indicators of the ward becoming in danger of being under-staffed and therefore tailor the number of available nursing staff as needed."
Dr Peter Carter, chief executive of the Royal College of Nursing (RCN), said: " We are encouraged that these guidelines have been created - following considerable and consistent evidence from the RCN of the danger to patients where there are too few staff.
"They underline what we already know - that a registered nurse caring for more than eight patients under these circumstances is a cause for concern - in many cases, considerably more nurses will be needed.
"Hospitals simply can't deliver what today's patients need if they are staffed for care from another century. Too often, staffing levels are allowed to yo-yo because of finances, and this puts staff under strain and patients at risk.
"For any patient to receive substandard care is unacceptable. Nurses will be hoping that once the full set of guidelines is completed, the NHS will never again be so vulnerable to short term financially driven decisions about patient care."
Peter Walsh, chief executive of patient safety charity Action against Medical Accidents, said: " More than eight patients per nurse on acute wards is definitely unsafe - so why shouldn't it be mandatory not to exceed that ratio?
"The Care Quality Commission should now use this guidance and include clear requirements on staffing in its standards and enforce them."
The final guidelines are expected to be published in July.
A Department of Health spokeswoman said: "There are over 5,100 more nurses on our wards since 2010 and in response to the Francis Inquiry we have been tough on insisting on compassionate care in our hospitals.
"We have increased the NHS budget in real terms and are clear that hospitals must balance their books whilst ensuring compassionate, quality care for all. We know this can and is being done - safe staffing levels lead to better care and can save the NHS money."
Jane Cummings, NHS England's Chief Nursing Officer, said: "I welcome the consultation from Nice and their expertise in reviewing the evidence.
"Each ward in each hospital around the country is different in size, number of patients, the type of patients and their needs.
"It needs a sophisticated approach, using hard evidence and local professional judgement to determine what staffing is right to provide the best care for patients in every setting. These Nice guidelines are a fundamental part of this sophisticated approach and I look forward to feedback from the public consultation."