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 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, leading experts to warn the NHS will struggle to cope with the busy winter period.
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.
Royal College of Nursing chief executive Janet Davies said the pressures on numbers meant nurses were being overworked and unable to provide the care they wanted to.
"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," she told the BBC Radio 4 Today programme.
"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.