The number of deaths due to errors is rising
More than 3,000 hospital patients have died because of errors by NHS staff in England over the past year, figures show.
Hospitals reported 3,645 deaths in 2007-8 from patient safety incidents, data from the Lib Dems showed.
The figure included those relating to problems with scans and tests and hospital infections.
It represents a 60% rise in the last two years, but experts said it was due to better reporting not worsening care.
Nonetheless, the Liberal Democrats, who requested the National Patient Safety Agency (NPSA) data from the government, said the health service needed to improve its performance.
Norman Lamb, the party's health spokesman, said: "Most of these deaths are avoidable and completely unacceptable.
"Along with very high standards in most hospitals, there are also areas of completely unacceptable practice.
"At the heart of this problem is weak management and indifferent political leadership.
"The NHS must get serious about improving patient safety."
And Andrew Lansley, the shadow health secretary, added: "While some of this increase may be down to improved reporting, these numbers are concerning."
Of the total number of deaths, 156 were due to errors in assessment, including diagnosis, scans and tests and 309 from the control of infections.
Nearly 500 were down to self-harm including suicide when patients were under supervision, while the rest were related to a range of other causes.
The NPSA reporting system relies on hospitals owning up to errors and in recent years has been encouraging the service to be more open.
It has seen the number of reports of errors rise by 50% in the last two years to over 800,000 a year, the majority of which cause no harm.
Dr Kevin Cleary, the NPSA's medical director, said: "This is a positive indication that there is an evolving and improving patient safety culture in the NHS."
A spokeswoman for the Department of Health said: "The NHS sees a million people every 36 hours.
"Unfortunately, as in any health service, mistakes and unforeseen incidents will occasionally happen.
"Only a tiny number of errors put patients at serious risk and the quality and safety of healthcare is improving all the time."