And some new patients seeking help are being sent to treatment facilities hundreds of miles from their homes to get care as a result of this so-called bed blocking.
Many who have been on acute psychiatric wards for nine weeks or more could be receiving more appropriate care elsewhere, researchers said.
The comments come as experts examined an unprecedented situation where around half of the acute psychiatric beds in one English county closed suddenly in May 2014.
The discovery of a fire safety flaw led to the temporary closure of a mental health facility in Cornwall.
This meant that over the space of just six days, 30 out of 54 acute psychiatric beds in the county were closed.
At the time 26 patients were receiving care at Longreach House in Redruth.
When the patients were being assessed to see what facility they should be taken to, only 10 were deemed to need an acute bed.
Meanwhile, none of the seven patients who had been on the ward longer than nine weeks needed a bed on an acute ward. But two needed to be admitted to a rehabilitation unit.
Following the retrospective analysis, the authors said that more emphasis needs to be placed on moving patients on from acute mental health units when they are ready.
They wrote: “Most patients on an acute ward longer than nine weeks may not need an acute bed, and more assertive efforts to move them to more appropriate settings will release pressure on acute units and may improve the personalised care for the patients in finding more appropriate therapeutic environments.
“More assertive management to move patients on from hospital after nine weeks on an acute unit may support patients in a less restrictive environment, release pressure on inpatient units and decrease the necessity of sending patients hundreds of miles from their homes when acutely unwell, which is distressing for them and their families and friends.”
Securing money to get patients in the most appropriate care can be “lengthy and bureaucratic”, they said.
This can lead to patients being kept on wards longer than necessary, which can lead to new patients being shipped “out of area”.
“What was noticeable in this emergency situation was that funds were urgently released for appropriate placements, whereas to get funding in normal circumstances can be lengthy and bureaucratic,” the authors wrote in the Journal of the Royal Society of Medicine Open.
“The prolonged time taken for funding assessments leads to inappropriately long stays in acute psychiatric wards for some patients with chronic needs resulting in new admissions of acutely unwell patients having to go to out-of-county hospitals.”
They added: “We conclude that greater emphasis and urgency needs to be placed on moving patients on from acute mental health units after nine weeks of admission.
“This can lead to more appropriate care for patients in less restrictive environments and reduce demand on acute psychiatric units and reduce the necessity and stress to patients and carers of acute admissions far from home.
“There needs to be more urgency in finding funding for such placements.”
The lead author of the study, Dr Richard Laugharne, a consultant psychiatrist from the Cornwall Partnership NHS Foundation Trust, said: “While presenting a stressful challenge to staff, the ward closure presented the opportunity to evaluate how the service dealt with a very unusual situation from which lessons can be learned.
“The prolonged time taken for funding assessments leads to inappropriately long stays in acute psychiatric wards for some patients with chronic needs.
“This results in new admissions of acutely unwell patients having to go to out-of-county hospitals.
“This is distressing for them and their families and friends.”
The Government has set a target for the elimination of inappropriate out of area placements in mental health services for adults in acute inpatient care by 2021.
Meanwhile figures published in the Guardian newspaper on Tuesday show that seven out of 10 specialist nurses believe that NHS services are inadequate to care for the growing number of youngsters with mental health problems.
A survey of 631 mental health nurses working in child and adolescent mental health services (Camhs) found that half believe services are inadequate and a further 20% believe they are “highly inadequate”.
The poll, undertaken by the Royal College of Nursing (RCN) for the newspaper, also found that 43% thought quality of care was getting worse for youngsters.
Samantha Nicklin, head of campaigns at the charity Rethink Mental Illness, said: “When on the one hand we have a chronic shortage of mental health beds, and on the other people are staying too long because of inadequate support elsewhere, then we have a lose-lose situation.
“We know that when it comes to supporting people with severe mental illness the least restrictive option is usually the most effective.
“While no one should have to leave hospital before they’re ready, we absolutely need a system that allows people to move out of acute care and into a more appropriate environment, such as supported housing, or care in the community.
“At the moment these pathways can be confusing and fragmented, and too often the community based services that people could move to are patchy and overstretched.
“The Government has committed to the Five Year Forward View on Mental Health which seeks to address these issues, so we need to see the recommendations that were made turned into action, with funding for services and better designed systems to really support people with mental illness on the road to recovery.”