Death rates at 12 NHS hospital trusts in England were alarmingly high last year, according to a new report.
Patient safety is also being risked because hospitals are "full to bursting", with many regularly breaching the 85% limit set in place to protect patients, according to the Dr Foster Hospital Guide.
The recommended bed occupancy level for hospitals is 85% but the analysis suggests most trusts operate closer to 90% for the vast majority of the year.
The report points out that quiet times of the year such as weekends and bank holidays distort the true strain on services.
The Royal College of Nursing said it revealed "a health care system under great strain".
In a statement the organisation added: "This is a highly credible report which should be taken seriously by everyone involved in health care.
"It is of great concern that so many hospitals are operating close to capacity."
Each of the 12 trusts highlighted in the report fell short on two of four mortality rate indicators - which include deaths after surgery and the deaths of patients who were admitted for minor ailments or low-risk conditions.
The report also suggests that a shortage of senior doctors working out of hours could be linked to higher mortality rates at the weekends.
Buckinghamshire Healthcare NHS Trust, The Dudley Group NHS Foundation Trust and George Eliot Hospital Trust in Nuneaton had high mortality rates three years in a row, according to the report.
Authors also alerted authorities to problems at the Mid Staffordshire NHS Foundation Trust - where the deaths rates were also high for a number of years.
The trusts highlighted have defended their records.
Kevin McGee, chief executive at George Eliot, said death rates had improved so far this year. While a spokesman for the Dudley Group said recent reports suggested a "significant and sustained improvement."
The Buckinghamshire Healthcare NHS Trust strongly denied that it had a higher than expected mortality rate.
Medical director Graz Luzzi said: "The title says it all. This is a hospital guide, but our care is also delivered at home, in the community and in a hospice. To be effective it is important to compare like with like."
Dr Foster also found that almost a third of hospital beds in England were occupied by patients who didn't really need to be there, with 3% of all hospital beds are occupied by people whose only diagnosis is dementia.
"These are people who have not been able to get the care in the community and as a result have ended up in A&E and put into a hospital bed - they really shouldn't be there," Roger Taylor, co-founder of Dr Foster, told Sky News.
Cambridge University Hospital NHS Foundation Trust was named Dr Foster's Trust of the Year for providing good quality and efficient care for patients.