Death rates at 12 hospital trusts in England were alarmingly high last year, according to an influential report.
The number of patient deaths are above expected levels at the 12 trusts, the Dr Foster Hospital Guide found.
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.
Figures shows that in 2011-2012 occupancy was running at 88% in midweek, while averaging 90% for 11 of the 12 months, excluding quiet periods including Christmas,
The Guardian, which has seen the report in full, added that the national level was over 85% for 230 of the 365 days of last year, and over 90% for 19.
Dr Andrew Goddard, the director of the medical workforce unit at the Royal College of Physicians, which represents hospital doctors, told the Guardian: “If you ask any doctor in this country they would say that the system is straining to burst; particularly in winter, but now it’s increasingly happening the rest of the year.
“Hospitals always seem to be full.”
The report, part of which has been seen by the Press Association, outlines concerns that there could be “another Mid Staffs” as hospitals are increasingly focusing on cost of care rather than quality of care.
Each of the 12 trusts 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”.
“These measures are to be used as a warning sign that poor-quality care may be leading to a higher-than-expected mortality,” the report states.
The authors continue: “With the rising demand for care and falling revenues, there are concerns that trusts will focus more (or exclusively) on cost of care rather than quality of care.
“Because of this, there is a fear that there could be another Mid Staffs. Hospital managers must ensure that they do not sacrifice one for the other.”
The report also suggests that a shortage of senior doctors working out of hours could be linked to higher mortality rates at the weekends.
However, University Hospitals Birmingham Trust (UHB) - one of the 12 trusts named as having higher death rates in two categories - raised concerns about the validity of the Dr Foster indicators.
Dr Dave Rosser, medical director at the trust, said: “The HSMR (Hospital Standardised Mortality Ratio) is widely considered to be unsuitable for use as a comparative indicator between hospitals.
“Dr Foster frequently changes the methodology of the HSMR which, in our opinion, further reduces its credibility as a comparator.”
He continued: “In our opinion, the mortality indicator relating to conditions of low clinical risk is deeply flawed.
“To illustrate, one of the patients identified by Dr Foster to be in this category was a patient admitted into one of UHB’s specialist services with a condition called toxic epidermal necrolysis, which is known to have a mortality in excess of 50%.
“Under Dr Foster’s methodology, this condition is classified as an allergy and therefore treated as ‘low clinical risk’. There are many similar examples.”
The report, which measures mortality indicators at 145 acute hospitals in England, also found that death rates at three hospitals have been consistently high for three years running.
Buckinghamshire Healthcare Trust, The Dudley Group Trust and George Eliot Hospital Trust in Nuneaton had high mortality rates three years in a row.