HMP Manchester was one of the prisons visited
Funding cuts are jeopardising prisoner safety in overcrowded jails in England and Wales, Council of Europe inspectors have said.
The Committee for Prevention of Torture visited jails in south-east England, and Greater Manchester and said the state of affairs was "to be deplored".
And they have called for routine strip-searching of young offenders to end.
The government says front-line work is protected from efficiency savings and it is creating extra cell space.
The CPT makes periodic visits to detention centres across the EU.
In November and December last year, it visited HMP Manchester; Woodhill, in Buckinghamshire; and Wandsworth, in south-west London, along with Maghaberry and Magilligan prisons, in Northern Ireland.
Members also visited Huntercombe young offender institution (YOI) in Oxfordshire and the Harmondsworth immigration removal centre.
In their report, they noted that 87 of 142 prisons in England and Wales were above "normal" occupancy levels, with too many inmates spending too long "locked in their cells with little access to any meaningful activities".
Of the three prisons visited, the CPT said all were operating at capacities "above what the Prison Service considers as good decent standard accommodation".
It said Maghaberry, the only high security prison in Northern Ireland, had 845 prisoners for an designated capacity of 670 but Magilligan was only about 80% full.
It called for UK authorities to "consider fresh approaches towards eradicating overcrowding as a chronic feature of the prison system".
They said the 3% year-on-year efficiency savings imposed on the Prison Service had reduced the prison working week to four-and-a-half days, with reduced staffing levels from Friday at midday.
"The CPT is concerned that the declared objective of holding all prisoners in a safe, decent and healthy environment is in fact being jeopardised by year-on-year efficiency savings," its report said.
It said this was a particular problem at Hunterscombe, where "juvenile inmates now spend most of their time between Friday midday and Monday morning locked in their cells", although the government contested this.
The committee also raised concerns over the growing numbers spending longer than a year in immigration detention.
In its written response, the government said it was building five new 1,500-capacity jails and expanding existing sites.
'No adverse impact'
It said evaluation had shown changes to working hours had involved "no adverse impact on conditions or treatment of prisoners".
With regard to Hunterscombe, the government said introduction of a new education contract ensured "a regime of daytime activities such as... domestic visits, religious services and gym sessions takes place over the weekend".
It added that it did not consider routine strip-searching there to be "disproportionate".
"It is used to safeguard individuals, other young people and staff in these establishments," it added.
It said the UK Border Agency tried not to hold immigration detainees for longer than necessary.
However, delays were caused by "last minute legal barriers, difficulties in obtaining travel documents and the non-compliance of the detainees themselves".