Fears have been raised about the government's handling of the NHS as figures show the emergency readmission rates to hospital are rising.
The number of people being readmitted after discharge has gone up by nearly a quarter since 1998, NHS figures show.
Campaigners said patients were being rushed out of hospital and that community services, which ministers have been promoting, were below par.
The government said the reasons for the rise needed to be investigated.
"It is imperative that the pressure to speed up an older person's discharge from hospital is backed up by services in their community"
Ministers have placed a big emphasis on developing services outside hospitals as they try to treat people closer to their homes.
This has come at a time when the NHS has been reducing the overall number of beds as patients spend less time in hospital because of better treatment.
But campaigners have long argued there has been insufficient investment in social care and NHS community services to cope with quicker discharges.
The figures, released by the NHS Information Centre, showed that there was a 22% increase in readmissions to 8.6 per 100 discharges among the 16 to 75 age group from 1998 to 2006.
For the over 75, the increase was even higher at 31% to 13.6 per 100 discharges.
Investment call
Gordon Lishman, director general of Age Concern, said: "The growing number of older people being readmitted to hospital indicates poor hospital discharge planning and has created a 'revolving door' in the NHS for some older people.
"Older people don't just need the right treatment while they are in hospital but also the right support when they go home.
"No-one wants to stay in hospital longer than they have to. But it is imperative that the pressure to speed up an older person's discharge from hospital is backed up by services in their community.
"This is a cause for concern and we need more research to help us understand the often complex underlying causes"
"Greater investment in services to help older people recover after leaving hospital is urgently needed or older patients' lives will be put at risk."
Health Minister Andy Burnham said research had suggested the rise in readmissions was not related to the standards of hospital care.
But he added: "This is a cause for concern and we need more research to help us understand the often complex underlying causes.
"We need to understand more fully what the causes of this long-term trend are including consideration of whether the changing age profile is a factor."
He also said guidance made clear that discharge must be properly planned, appropriate community services should be in place, and patients should not be discharged until it was safe to do so.
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