One in ten of all patients admitted to hospital is harmed by complications, half caused by medical mistakes of some kind.
The cost of the extra days in hospital needed is running to at least £1bn a year, said the research team from University College London.
A leading doctor said that the profession was failing to "learn from its mistakes".
The study looked at more than 1,000 medical and nursing records at two hospitals in London, then expanded these results to produce national predictions.
Researchers found that almost 11% of patients had suffered some sort of adverse event - some more than one.
In approximately 50%, the researchers judged that hospital staff should have prevented them happening.
Contributing to death
A third of the events led to "moderate or greater impairment" - in 19% of cases, the decline in their health as a result was permanent, and in 6%, the adverse event contributed to death.
The figures contained in the report were first aired last year by the Chief Medical Officer Professor Sir Liam Donaldson, but have now been published in full in the British Medical Journal.
Professor Sir George Alberti, president of the Royal College of Physicians, wrote in an accompanying editorial that the "blame culture" in society put doctors and nurses off admitting mistakes.
He said: "If we are to learn from mistakes then we need to know about as many as possible so that corrective action can be taken.
"The main causes of adverse events relate to operative errors, drugs, medical procedures and diagnosis. Each of these is amenable to prevention."
However, he also admitted that mistakes were inevitable because there were too few medical staff, who were forced to work too quickly to cope with intense demand.
He told BBC News Online: "It's still the case that nobody knows exactly how many of these events are happening in UK hospitals each year.
"This study only covered two hospitals, so their figure is only an estimate.
"The Department of Health should be funding the collection of more data."
The issue of medical mistakes has been highlighted recently by cases in which wrongly administered drugs killed two patients, and a three-year-old girl died after being given nitrous oxide gas instead of oxygen.
The author of the report, Professor Charles Vincent, said: "Adverse events are a major source of harm to patients and a major drain on NHS resources.
"Whether they are major events that are traumatic for staff and patients alike, or minor events that are frequent but go unnoticed in routine clinical care, together they have massive economic consequences for the provision of healthcare to use all."