Inspectors highlighted serious failings at Basildon
Twelve NHS hospital trusts in England are "significantly underperforming", a report has said, despite eight recently having been rated as good or excellent.
The report by monitoring body Dr Foster also said 27 trusts had unusually high death rates.
But the Care Quality Commission, which has issued its official ratings within the past month, said it saw no need to step in at the moment.
Some trusts have hit back at Dr Foster, describing its data as misleading.
University Hospital of South Manchester NHS Foundation Trust described the findings as a "grossly misleading assessment". It said the report had been met with "shock and disbelief".
In a statement, the trust said: "In their last report, Dr. Foster rated UHSM in the top ten safest hospitals in the country, based on low mortality rates.
"Mortality rates at UHSM are even lower this year, yet the same hospital is now being catapulted to the bottom of the National league table."
A statement by Mid Yorkshire Hospitals NHS Trust described the Dr Foster report as "extremely confusing".
Its chief executive Julia Squire said: "People can be confident about the care we offer and our focus on patient safety."
Variations in care
The part-private, part-NHS Dr Foster Hospital Guide rates hospital trusts from one to five based on analysis of a range of "safety indicators", including errors in surgery, deaths, infection rates and staffing levels.
In this year's report 12 received the lowest score.
They include University Hospitals Coventry and Warwickshire; Weston Area Health Trust; South London Healthcare Trust; Tameside Hospital Foundation Trust; University Hospital of South Manchester and St Helens and Knowsley Hospitals Trust.
Roger Taylor: "There is no right way to measure a hospital"
Mid Yorkshire Hospitals Trust; Blackpool, Fylde and Wyre Hospitals Foundation Trust; and Hereford Hospitals Trust are also among the 12, which are completed by Basildon and Thurrock; Lewisham; and Scarborough and North East Yorkshire.
Of these St Helens and Knowsley was rated excellent by the CQC and three others - Mid Yorkshire, Weston Area and Scarborough and North East Yorkshire - were rated fair, with seven of the remainder achieving good ratings.
South London Healthcare was formed by a merger of three trusts, one of which was rated good in the latest Care Quality Commission findings.
Overall, the Dr Foster report said hospital death rates in England had fallen by 7%, but quality of care continued to vary greatly across the country.
It cited more than 200 cases where items such as swabs had been left inside patients' bodies after surgery and more than 5,000 instances where people who were admitted with low-risk conditions died in hospital.
Roger Taylor, director of Dr Foster, said: "No hospital in the world would claim to be free of these kinds of errors, but what has been learned over the years is that recording what is happening, and making that information generally public, is the best way to start tackling the issues."
"The vast majority of patients experience high-quality, safe and effective care and the CQC's recent survey showed that 93% of patients rate their overall care as good or excellent."
The Conservatives highlighted what they said was the "extraordinary" contrast between Dr Foster's findings and the CQC's own assessments.
Shadow health minister Stephen O'Brien told the BBC: "Clearly there needs to be a massive overhaul in the way that the inspection regime is working, because... the failed system of self assessment can't carry on and we need to rip up the performance indicators - above all stop the tick-box targets."
Liberal Democrats shadow health secretary Norman Lamb said: "We were told that the mid-Staffordshire case was a one-off scandal.
"At the time I called for a public inquiry and it was rejected then."
He added: "Now the case is overwhelming and we appear to have a system which seems to be able to deliver 'paper-safety' but not real 'patient safety'."
On Thursday, the Care Quality Commission sent a task force into Basildon and Thurrock NHS Trust in Essex after it uncovered major lapses in hygiene and unusually high death rates.
Just 24 hours later, the chairman of Colchester Hospital University NHS Foundation Trust was sacked after inspectors found it had consistently failed to improve waiting times and had death rates 12% higher than expected.
Health Secretary Andy Burnham reacted by ordering an urgent review by regulators to find out whether any other trusts required "immediate investigation".
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