Jails run in the name of Her Majesty's Prison Service on the Cayman Islands are “barely fit for human habitation”, UK inspectors have found.
Custodial facilities in the British overseas territory were "dark, decrepit, chaotic and dirty" and the UK should take steps to ensure regular inspections take place, Her Majesty's Inspectorate of Prisons (HMIP) said.
Illegal drug use was rife in one of the prisons inspected, while HMIP staff discovered children sharing cells with adults were put at risk of predatory sexual behaviour.
Inspectors visited HMP Northward, which holds around 200 men, HMP Fairbanks, which held just four women, and a number of custody suites and court cells, after receiving an invitation from Cayman Islands governor Duncan Taylor.
The squalid conditions described by inspectors are a sharp contrast to life outside the prison walls in the Cayman Islands, which is known as a tax haven and a luxury holiday destination.
Chief inspector of prisons Nick Hardwick said: "What we found was troubling and concerning, particularly given that the prisons operated in the name of Her Majesty's Prison Service.
"Whatever one's view of the role of imprisonment, most people would expect prisons to hold prisoners safely and decently, and to ensure prisoners leave as better people than when they came in. Northward and Fairbanks fall well short of this standard."
He added: "It is difficult to see how this will be achieved without significant investment and a radical review of governance structures."
The Cayman Islands has more registered businesses than people, with a population of around 60,000 covering a mix of more than 100 nationalities.
Her Majesty's Cayman Islands Prison Service falls under the jurisdiction of the territory's Ministry for Internal and External Affairs.
The HMIP report found that living conditions for prisoners at Northward were "unremittingly poor with most cells barely fit for human habitation".
It said: "All wings at Northward were dark, decrepit, chaotic and dirty, with an oppressive and intimidating atmosphere and a strong smell of cannabis. Communal areas were grubby, bare and devoid of equipment."
Inspectors found that male juvenile prisoners were "at serious risk of harm", while security arrangements were poor.
"We found evidence of male juveniles put at risk of predatory sexual behaviour and recruitment into gangs," the report said.
High levels of illegal drugs were found at Northward, which were easy to obtain, while the prison took no action to prevent the flow of drugs into the facility with no targeted drug testing.
HMIP said many buildings at both prisons should be knocked down, while the remaining facilities should be renovated.
The UK should also sign up the Cayman Islands to the Optional Protocol for the Convention Against Torture (Opcat) - a treaty that requires ratifying countries to ensure torture or degrading behaviour does not occur.
The UK is signed to the treaty but it does not apply in the Cayman Islands, which means that areas of detention are not subject to regular independent monitoring.