There has been a large and avoidable rise in the number of overnight stays, which cost the NHS £330m annually, a report has said.
The study, from independent think-tank the Nuffield Trust, said the increase was “unsustainable” and pointed to large variations between hospitals in how many patients were admitted.
It calculated there was a 12% leap between 2004/05 and 2008/09 in the number of patients going into hospital as an emergency - resulting in around 1.35 million “extra” admissions.
The number of “costly and frequently preventable” emergency admissions, mostly through A&E, rose from 4.4 million in 2004/05 to 4.9 million in 2008/09, the study said.
Around one in three of all hospital admissions in England are emergencies, costing the NHS some £11 billion a year - one of the most expensive areas of the health service.
The report said the cycle would need to broken in the future through creating better out-of-hospital care and preventing patients becoming sick in the first place.
Nuffield Trust director Dr Jennifer Dixon said: “Our hospitals are over-heating and are on an unsustainable path in which they are treating patients at great cost to the NHS and to patients themselves.”
The College of Emergency Medicine said it did not accept that admissions for fewer than 24 hours were unnecessary.
It said many emergency patients require access to diagnostics (such as blood tests and X-rays) and a period of observation to ensure the decision to discharge the patients home “is informed and safe”.
Health Secretary Andrew Lansley said: “We need a more integrated approach to NHS care. It’s better for patients and it’s also cost effective. It’s an approach which would ensure patients are not treated like drones in a production line but are given the best care at every point of their journey.”