Nine out of 10 hospitals are failing to meet their own targets for safe levels of nurses on wards, according to a report.
Experts say staff are "at breaking point" and that the nursing crisis is one of the worst the NHS has ever seen.
And they warned that, with another winter crisis looming on the horizon, hospital wards will come under greater strain.
Nurses are so overstretched they can only carry out "absolutely essential" tasks, and some feel they are giving "sub-standard care", it has emerged.
An analysis by the Health Service Journal (HSJ) of 232 hospitals in England found that 207, or 90%, were unable to meet safe levels during the day, while 81% could not hit targets for night cover and some 79% missed both quotas.
It marks a decline since January this year, when 85% of hospitals were short-staffed during the day.
Jacqui Graves, head of health and social care at Macmillan Cancer Support, told the Press Association the figures are "worrying and shocking".
She said: "At the moment what we see is that staff are pretty much at breaking point.
"It comes at a time when nurses are already under a lot of pressure - partly because of the winter crisis that always comes when we have got a lot of elderly people in hospital, but also because there is staff shortages and because we are getting to a point now, particularly in cancer nursing, where a lot of nurses are going to be retiring in the next five to 10 years. So there is also another crisis looming."
She said nurses no longer have the time to spend five minutes with their patients to find out "what makes them tick" and give them the emotional care that can be so crucial to someone's recovery.
She said: "Clearly it will affect patients if there isn't that number of nurses to be able to give them that better experience.
"But also, from a staff point of view, they are at the point where they are feeling 'I don't have the time to be able to give the standard of care that I want'. And that is really frustrating for most nurses.
"They want to care, but if you are consistently having to give care that you think is sub-standard then clearly that has a real knock-on effect in terms of your own personal satisfaction.
"The reality is that they are only able to do the things that are absolutely essential.
"It is probably one of the biggest crises we have seen."
The HSJ's findings, based on August figures published by hospitals each month under measures introduced following the Mid Staffs inquiry, come as the NHS faces increasing pressure.
Earlier this month the health service missed a raft of key targets for A&E waiting times, cancer treatment and ambulance responses.
Separate research last week suggested nurses are under such pressure they cannot guarantee safe care for their patients.
The Nursing Times survey of almost 1,000 nurses found eight out of 10 are under more stress at work than they were 12 months ago.
Britain is struggling with a nationwide shortage of nurses, and Chancellor George Osborne last month announced that student nurse bursaries worth up to £20,000 will be scrapped.
Janet Davies, chief executive of the RCN, blamed low nursing levels on Government cuts.
She said: "When a ward or a community team does not have enough nurses, it can be harder to meet the needs of patients, harder to recognise deterioration and harder to manage conditions in the long term.
"It doesn't take much to tip services over the edge, and the NHS could be very vulnerable to a bad winter and any extra pressures."
Shadow health minister Justin Madders said: "These figures illustrate the scale of the nurse staffing crisis now engulfing the NHS.
"The Government's cuts to nurse training places have left hospital wards dangerously under-staffed, forcing NHS bosses to waste huge amounts of money on expensive agency staff."
Christina McAnea, head of health at Unison, the UK's largest healthcare trade union, said: "Ministers should be hanging their heads in shame. Government policies are heaping untold pressure on to nurses and proposals to scrap the nursing bursary will make a bad situation much worse."
A Department of Health spokeswoman said: "Staffing is a priority - we've put almost 8,600 additional nurses on our wards since May 2010 and there are 50,000 nurses currently in training."
She said the Government is clamping down on expensive agency fees and making back-office savings.