Nine out of 10 hospitals are failing to meet their own targets for safe levels of nurses on wards, according to a report.
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.
The Royal College of Nursing (RCN) said low staffing levels made caring for patients far harder, and warned the NHS could be vulnerable to a winter crisis.
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 that 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.
More than half said they "rarely" or "never" had either sufficient staff or time to ensure safe care for patients, while a third said their ward or team was "always" short-staffed.
Britain is struggling with a nationwide shortage of nurses, and 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."
She added: "Nurses are committed to their patients and will continue to grapple with increased demand, heavy workloads and extra hours to provide the best care they possibly can.
"But if the NHS has a chance of keeping up with demand it needs to think clearly about how it retains, incentivises and values its hardworking staff."
She told the BBC Radio 4 Today programme nurses are being overworked and unable to provide the care they want to.
She said: "Our nurses are telling us that they are really exhausted at the end of their shift, often having to stay late, and really upset if they can't provide the care they know they want to do.
"They do get very concerned because they know what they should be doing for patients and if they can't it's really upsetting."
She said the health service was now paying the price for previous cutbacks to the nurse training programme.
"We went through a period of time when we were trying to save money, we cut posts, we didn't train enough people and we are still feeling the effect of that," she said.
Labour shadow health minister Justin Madders also blamed the crisis on Government cuts.
He 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 understaffed, forcing NHS bosses to waste huge amounts of money on expensive agency staff.
"This is impacting on patient care and leaving important tasks being left undone.
"We cannot allow this situation to be tolerated any longer. Ministers need to urgently ensure there are enough staff on hospital wards to deliver safe care. Nurses are the lifeblood of a successful NHS."