Prisons are struggling to cope with the increasing number of elderly, sick and disabled people behind bars, a Prison Reform Trust report says.
It reveals that those aged 60 and over are the fastest growing population in English and Welsh prisons, with an increase of 146% between 2002 and 2014.
Those aged 50-59 are the second fastest-growing group, says the report.
The government said it was "committed to providing suitable facilities".
The report comes as Prisons Minister Andrew Selous is due to give evidence on older prisoners to the Commons justice select committee.
'Less safe and less decent'
Longer sentences mean more people are growing frail in prison, but high rates of social and support needs are unmet, says the trust.
Two in five prisoners over 50 now have a disability of some kind, the report added.
Trust director Juliet Lyon said: "In the last few years, prison has been reduced to a punitive holding operation for people growing older and sicker behind bars.
"Prisons are less safe and less decent than they were even a year ago when we published our last report.
"An incoming administration of government in May 2015 must not accept this deterioration in prison standards and conditions as the new normal."
Although the prison population stands at around 84,500 in England and Wales - 20% higher than it was 12 years ago - the National Offender Management Service has had to make more than £700m in savings over the last three years and is expected cut a further £149m in 2014-15.
The proportion of prisons whose performance is "of concern" or "of serious concern" according to the National Offender Management Service has risen from 13% in 2012-13, to 23% in 2013-14.
Prisons minister Andrew Selous, said: "We're committed to providing suitable facilities and ensuring individual healthcare needs are met, as well as working alongside charities such as Age UK and Recoop to ensure older offenders are rehabilitated.
"From April next year, local authorities will be responsible for care and support for prisoners meaning that, for the first time, we will have a social care service working alongside the healthcare already provided in prisons by the NHS."