The prison service and local authorities are failing to plan for a rise in elderly, ill and frail inmates, a new report warns.

Many older jails, such as Winchester which dates from the mid-19th century, are ill-equipped for prisoners in wheelchairs or with mobility problems, the assessment by two watchdogs found.

HM Inspectorate of Prisons and the Care Quality Commission flagged up concerns about a lack of support overnight - with some inmates unable to summon assistance after falling.

In effect a "postcode lottery" has developed where prisoners can receive a poor, satisfactory or very good service based on which facility they are sent to, according to the joint review of social care provided in prisons in England and Wales.

It said the increasing prison population, coupled with longer sentences and sentences being given for historic offences, have contributed to its "reshaping".

As at December 2017, the number of people in prison aged 50 and over was 13,522 - representing 16% of the total adult prison population.

Projections indicate that the number of individuals aged 50 and over held in custodial settings is likely to increase.

The report said: "We are not convinced that there is adequate consideration of what will be required in the very near future, such as the obvious needs that will flow from the projected growth in the older prisoner population.

"This, in our view, represents a serious and obvious defect in strategic planning."

HMIP and the CQC noted that there have been some improvements in care for older and disabled prisoners since the introduction of new legislation.

Some prisons show good practice and there are many caring staff and fellow prisoners, the watchdogs said.

But the report highlighted the absence of a comprehensive national strategy for the provision of social care in jails.

It said the ageing population within prisons, coupled with increasing frailty and incidence of dementia, has accelerated the need for prisons to address social care needs.

In addition, a significant proportion of prisoners also have learning disabilities, autism, mental health disorders or difficulties which may inhibit their ability to cope with life behind bars.

Chief Inspector of Prisons Peter Clarke, who previously floated the idea of secure "old people's homes with walls", said: "Prisons were designed to accommodate physically fit and mentally stable individuals, with prison life being arranged to address the needs of the many.

"Prisoners with social care needs - unable to fully care for themselves, needing help in getting around the prison or in participating socially - are at a significant disadvantage."

Professor Steve Field, chief inspector of general practice and integrated care at the CQC, said: "As the prison population ages and becomes more likely to develop new conditions while in prison, it is extremely important that services are equipped and managed in a way that offers the high-quality care that everyone has the right to."

The responsibility for social care in prisons falls on 58 English authorities with jails and five in Wales.