These shaven-headed and heavily tattooed prisoners are locked up for 23-and-a-half hours a day. They never see sunlight, there are no family visits, nor are there recreational spaces or rehabilitation programmes.

This is life inside the world's toughest jail in El Salvador. These men who have committed crimes evil almost beyond comprehension have little hope of ever being released.

Built by the Salvadoran government to imprison 'high-ranking' members of the country's main gangs, the Terrorism Confinement Centre (CECOT) is a prison like no other and can hold up to 40,000 inmates. 

In a nation where the notorious Ms-13 and Barrio 18 gangs have wreaked deadly havoc, the prison has boosted President Nayib Bukele's popularity and success in the polls.

Critics of the president describe it as a 'black hole of human rights'. But after broadcaster Richard Madeley visited the £85million jail in Tecoluca, questions have been raised over whether Britain can learn from one of the world's harshest prison regimes.

Routinely described as being at breaking point, the UK's system has for years been plagued by overcrowding, attacks on prison staff, absconding inmates, staff-inmate relationships and the scourge of drones smuggling in phones and drugs.

Earlier this year, the Government announced plans to introduce US-style 'supermax units' to hold the most violent terrorists. The UK currently holds its most dangerous inmates in separation centres but calls are growing for stricter conditions.

Britain's most secure prison is Belmarsh's High Security Unit (HSU). With its own 20ft perimeter wall, electric gates opened remotely by central control and rigorous searches, the 'prison within a prison' holds the worst inmates in the country.

But even here, jihadis, rapists and murderers are afforded the luxuries of being allowed to play pool, spend hours of the day outside their cells, chatting to fellow inmates or going to the gym and library. 

Former prison governor Ian Acheson told the Daily Mail: 'The way we manage very dangerous people in this country who are as risky as some of the gang members Madeley saw is not currently sufficient for the threat they pose to frontline prison officers and other offenders.'

Mr Acheson said that the current system 'appeases' inmates who want to 'cause lethal harm', explaining: 'That has led to nearly lethal attacks on officers or the murder of inmates in some of our most controlled units.'

He added: 'We have to have a 'supermax' capability where the emphasis is on control first, not notions of rehabilitation that are simply exploited.'

Until recently, El Salvador had the highest murder rate in the world, with 106 homicides per 100,000 people. This has plummeted to just 1.9 in 2024 after a sweeping crackdown on violence and gangs.

In El Salvador, inmates are serving sentences from 60 to more than 1,000 years, while in the UK Labour has faced accusations of 'soft justice' after launching an emergency early release scheme to battle with record jail population levels in 2024.

Mr Acheson said it was a 'remarkable public safety achievement' that El Salvador's murder rate had plummeted.

But he added: 'That said, when you incarcerate 2.6 per cent of your entire adult population, you will reduce crime on the streets. The question isn't whether mass incarceration works in the short term — it's whether it's sustainable or desirable.

'But it couldn't and shouldn't work here. Cultural and societal differences matter. Just as Norway's hyper-liberal prison model relies on Scandinavian social cohesion we don't possess, El Salvador's brutal regime depends on a public desperation about gang violence that Britain hasn't experienced.

'Strip men to their underwear, shave their heads, deny them human contact and medical care, and you don't reform them — you warehouse rage. When they eventually leave, you've created exactly what you claimed to be protecting society from.'

'What we need here is a prison experience where retribution for harm done is meaningful and it is safe enough for rehabilitation to be a realistic possibility. We've all but lost that in jails awash with violence, indolence, drug addiction, staff shortages and despair.'

What are the cells and conditions like? 

Conditions inside the prison are unlike anything in the UK system.

In El Salvador's mega jail, more than 80 inmates sleep in metal bunks, stacked four-storeys high with no mattresses or sheets, inside vast concrete cells.

They never see sunlight, while the bright artificial lights are never turned off even during the night. 

Temperatures inside the cells can reach 35C during the day and the inmates are only allowed out for 30 minutes a day for brief, heavily-guarded exercise inside. 

These incarcerated men will never smell fresh air or see natural daylight again. 

Prisoners, who only wear boxer shorts and must have shaved heads, have nothing to do while sat in their cells for 23 and a half hours. They are not allowed books, magazines, newspapers or screens.

They must eat their meals inside their cramped cells. There are two sinks for prisoners to wash in and two toilets, which inmates must use in front of everyone else.

They are only permitted to scuttle out of their cages, shackled hand and foot with heads bowed low, for a small number of reasons - such as during cell searches and medical examinations.

At HMP Belmarsh, the HSU is a 48-cell facility spread across two floors and split into four 'spurs'. 

At one stage, Britain's most violent prisoner Charles Bronson - who has spent more than 50 years behind bars - had an entire spur to himself amid fears he would explode into violence and cause a riot. 

Each high-risk inmate has their own cell and is checked on by the governor every day. 

Cells are about 6ft wide by 10ft long, with a small window covered by wire mesh offering a dull view of a wall outside. 

Deep within the HSU lies an isolated cell known as 'The Box', which has no bed, sink or toilet. 

Britain's most dangerous criminals are unlocked from their cells at 8am every morning. They are then allowed to exercise for an hour and have time to socialise, including playing pool, going to the gym or engaging in reading groups.

But one thing that Belmarsh HSU has banned, which other separation centres in the country haven't, is cooking, as prison bosses feared it could lead to attacks with oil or makeshift weapons.

Prisoners are locked back in their cells for lunch at 12pm for around an hour and a half.

They are then released for some more association time in the afternoon and locked up again at 4.30pm. They are then sent back to their cells until 8am the next morning with no evening entertainment.

What food do prisoners get? 

At the CECOT, inmates are given three meals a day in their cells – of rice and beans, pasta and a boiled egg. 

Sat on the floor, they must eat this with their hands. Forks, knives and spoons are banned amid fears they would use a utensil as a weapon.

Their water is rationed by the guards who hand it to them.

At the HSU in Belmarsh, inmates are served three meals a day based on a rotating weekly menu.

Prisoners must get a minimum of five choices for the lunchtime or evening meal, including at least one substantial hot meal per day.

Inmates always have access to drinking water.

Unlike other separation centres at some of Britain's worst jails, the HSU is the only prison where inmates are banned from cooking. This is amid fears these criminals could use cooking oil and utensils as weapons.

Southport killer Axel Rudakubana hurled scalding water over a prison officer while being held inside the HSU. It is not clear how he got hold of the boiling water.

At other maximum-security prisons, including HMP Wakefield - dubbed 'Monster Mansion' - prisoners have been offered Christmas Day menus, including quorn roasts and festive vegan slices. 

What freedoms do inmates get?  

At El Salvador, the answers is simple. Nothing. There are no workshops, libraries, opportunities to learn kitchen skills and no visitors. 

This is not for rehabilitation, this is permanent exile from society.

Prisoners are confined to their cells for 23 and a half hours a day and must only whisper. They are not allowed books, magazines, newspapers or screens. 

'They just sit on their bunks, day in, day out, and the prison lights stay on 24/7, never dimmed,' Madeley said on his visit. 'All will die in this prison. It's a living death.' 

The exceptions are occasional motivational talks from prisoners who have gained a level of trust from prison officials.

Prisoners sit in rows in the corridor outside their cells for the talks or are led through exercise regimens under the supervision of guards.

The prison's dining halls, break rooms, gym and board games are for guards.

At Belmarsh's HSU, prisoners are afforded several luxuries.

They have access to a gym, libraries, social visits, legal visits, religious services, reading groups - and can even play pool with one another.

The only thing people on HSU don't have is work activity. 

Ex-Governor Mr Davis also revealed that there could be half a dozen HSU prisoners exercising together, explaining: 'It wasn't total isolation.'

He added: 'You would walk on the unit and they would be playing pool or they've got their little wing library. They would be spending their time talking and sitting around. I would just go and have a conversation while they're playing pool.

'They appreciated that you would come up and talk to them and spend a bit of time. With the special smaller units, you have that ability to do that.

'They had all the same facilities as the main prison, but it is just totally separate.' 

How are the prisons guarded? 

El Salvador's CECOT jail is an isolated fortress, nestled in the heart of the mountains around an hour and a half out of the capital.

Anyone wanting to enter is stopped at a checkpoint patrolled by armed guards around a mile away from the prison. Here, bags are intensely searched and vehicles are checked.

Before reaching the main gate, there is another similar checkpoint and mobile phone signal is non-existent. 

The prison is secured by a  nine-metre concrete wall, with three metres of electrified fencing.   

There are more than a thousands security personnel working at the prison, including guards, police and military patrolling watchtowers, corridors and cells.

Guards are heavily armed and masked to conceal their identities.

Meanwhile, the HSU is often described as a 'prison within a prison', owing to its separate unit within the walls of Belmarsh.

Prison officers at the HSU must be highly trained as they are dealing with the most dangerous prisoners in the country.

To prevent guards from becoming too familiar with inmates, HSU officers can only work in the unit for three years before being rotated back to the main prison. 

Questions have been raised over the level of protection for guards in Britain - including the HSU - after a spate of attacks.

Manchester Arena bomb plotter Hashem Abedi Abedi attacked a prison officer in May 2020 with two other convicted terrorists, Ahmed Hassan and Muhammed Saeed, inside the HSU.

The Belmarsh attack sticks in the memory of Mr Davis, who was governor at the time of the sickening attack on custodial manager Paul Edwards.

'It was a cowardly attack,' Mr Davis tells the Daily Mail. 'The custodial manager had his back to the door and was on his computer. The three of them were coming off the exercise yard and rushed in and attacked him.'

Bloodied Mr Edwards was left with lacerations to his scalp, had a bruised back and rib cage and suffered lasting damage to his hearing.

The HSU at Belmarsh is designed to control prisoners effectively, but it can also take its toll on staff.

'They are professional people, they have to be hand-selected, they receive psychological support and have meetings just to make sure everything is OK because it's quite an intense atmosphere,' Mr Davis says.

Who are the inmates? 

Notorious inmates including Islamic extremist Abu Hamza, Britain's most violent prisoner Charles Bronson and Soham child killer Ian Huntley have been caged in Belmarsh's HSU since it was opened in 1991. 

The HSU at HMP Belmarsh became operational in April 1991 and was initially designed to house high-profile IRA prisoners.

However, in more recent years, it has held some of the worst offending prisoners in the country including KGB agents, murderers and jihadis.

Manchester Arena bomb plotter Abedi was moved back to the Belmarsh HSU after allegedly attacking three prison officers at HMP Frankland by throwing cooking oil over them and stabbing them with makeshift weapons.

Abedi had previously been transferred out of Belmarsh, in Thamesmead, south-east London, after attacking a prison officer in May 2020 with two other convicted terrorists, Ahmed Hassan and Muhammed Saeed, inside the HSU.

Southport killer Axel Rudakubana also allegedly attacked a prison officer at HMP Belmarsh last May by pouring boiling water over them.

High-profile prisoners who were housed in the main jail include Wikileaks founder Julian Assange and far-right activist Tommy Robinson.

In CECOT, the vast majority of inmates are high-ranking and violent members of El Salvador's gangs, MS-13 and 18th Street. 

They have raped and tortured, murdered and mutilated, cut bodies to pieces and strewed them around the streets to strike terror in the neighbourhoods they controlled. 

In March, the US government deported hundreds of Venezuelans to CECOT, claiming they were part of the Tren de Aragua crime group.