A tour of the jail within a jail that houses Britain's most dangerous convicts

Mark Hughes is the first newspaper journalist to be allowed inside Belmarsh's High Security Unit

Cell 12 on wing three is about 6ft wide by 10ft long. A small window covered by wire mesh offers the dull view of a wall outside. There is a small television on a plastic desk in one corner and a metal toilet in the other.

Only the long, specially-adapted, lever tap handles give any indication as to the identity of the occupant. For the past six years this has been "home" to Abu Hamza, the notorious Muslim cleric. He is one of just nine prisoners held in Britain's most secure prison.

Holding the country's most dangerous criminals, HMP Belmarsh's High Security Unit (HSU) is a prison within-a-prison. And, until now, almost nothing has been known about it. Even within the main jail, most of the 843 prisoners have no idea what goes on inside.

But this week The Independent became the first newspaper to be granted full access to the HSU and allowed to speak to the men whose job it is to guard the country's most dangerous criminals. We saw the cramped living conditions and tedious regimes that men such as the 21/7 bombers and Bilal Abdullah, the man behind the Glasgow Airport attack, have experienced.

To get inside the main prison I had already negotiated 15 gated doors and had my fingerprints scanned. On arrival at the HSU – a windowless, grey concrete building opposite the prison's recently-built five-a-side football pitch – the security checks began again.

Surrounded by CCTV cameras in a small carpeted reception area – the only carpet in the block – I removed my shoes and belt and put all my belongings through an X-ray machine. I walked through a metal detector and a was given a body search – the lining of my jeans, the soles of my feet and inside my mouth were all checked.

This security is not just for visitors – the prison guards must go through the same search before they enter.

At the end of the reception area is a red iron gate. Passing through this door involves at least a four-minute wait, as it can only be unlocked by staff in the control room who check people's identity using remote cameras which zoom in to study their faces.

Once through you are faced with four more doors, each leading to a different part of the unit. No two doors in the unit can be opened at the same time.

The HSU is on two floors and is split into four "spurs". Each one has 12 single-occupancy cells. Built alongside the main prison in 1991, the HSU was originally used almost exclusively to house IRA prisoners. But since then it has held KGB agents, al-Qa'ida terrorists and even Charles Bronson – Britain's most violent prisoner – who had a whole spur to himself.

But while Bronson was deemed too dangerous to mix with others, the men currently held in the HSU are not there because of any physical risk they pose. For the most part it is their notoriety which earns them a place there.

"We get a lot of high-profile prisoners, and prisoners who have the means and capacity to escape," one of the the HSU managers explained. "The type of prisoner we have here is a lot different to the type of prisoner in the normal prison. The prisoners here have the means and ability to achieve the results prisoners somewhere else would not."

That applies to Curtis Warren, who is currently being held in the HSU. He is a gangster, a drug trafficker and was once Interpol's most wanted man.

The fear around Warren, a powerful and influential criminal on the outside, is that he would be both of these things inside the prison were he to mix with other prisoners. And, despite the fact that Belmarsh has never had a prisoner escape in its 19-year history, he would also be a possible escape risk.

A different fear exists around Abu Hamza: that he would use his preaching to radicalise other Muslim inmates. He cannot do that from the confines of the HSU. Indeed, he cannot do much.

The prisoners here have a similar regime to the inmates in the main prison except that they are not allowed to work – prison jobs include packing teabags and cleaning.

They are in their cell for 12 hours and out for 12. The day starts at 8.10am. They are given 20 minutes for breakfast, an hour of outdoor exercise, an hour to use the gym and have to clean the wing for half-an-hour a day. The rest of the time spent out of their cell – five hours – is "association" time.

During this period they can chat to one another, play pool or table football, watch television, or use the rowing machine or exercise bike which sit on the wing. There is also a laundry and a small shower cubicle.

While it may not sound a particularly taxing regime, it is far from stimulating. And it is certainly not the holiday camp which some commentators would have you believe. The area is desperately cramped and uncomfortably warm. During their five hours of association, the HSU prisoners cannot leave the confines of their spur.

The only outside areas are two surprisingly large exercise yards, surrounded by high fences topped with barbed wire and metal mesh for a roof.

Passing one yard I saw two men slowly pacing around the perimeter. They were being watched by four prison guards. At that moment an alarm went off. Unlike the perception most people have of a prison, it was not an audible siren but a coded message via the guard's radio. We were told we were not allowed to move. As we watched the two men exercise, one of them shared a joke with a guard.

It was at this point an officer warned me that those men, despite already being in the HSU, had been categorised as "exceptional risk" prisoners and were not allowed to mix with anyone except each other. He added: "Those two men are two of the most powerful people in prison in the whole of Europe."

If the HSU sounds like an additional punishment, it is not meant to be. In fact the prison has gone to surprising lengths to keep its most dangerous men happy. Abu Hamza's disability, the fact that he has no hands (he is not allowed his hook in prison), has been catered for. Two cells in the HSU have been kitted out with special taps, shelves and clothes pegs.

Staff are warned against becoming too friendly with the inmates. Officers on the HSU are only allowed to work there for three years before being moved back to the main prison. They are also warned against sharing any personal details with the prisoners.

It is for this reason that the HSU manager asks for his first name not be be published. Senior officer Murray explained: "Our staff here are trained to spot manipulation and conditioning. We don't like staff to become over-friendly because it can get to the point where a prisoner has a member of staff in his pocket and can manipulate him for favours or telephone calls."

Precautions are taken to ensure that prisoners cannot hide anything in their cell. Every few months men are moved to a different cell and the cells are searched. This is why Hamza has two.

The day I visit, everything is calm in the HSU. Inmates, in their prison-issue, maroon jogging-bottoms, use the gym under the watch of the guards.

But it is not always like this. The prison governor Phil Wragg recalls an incident where the HSU inmates refused to return to their cells and had to be forcibly restrained. But he dismisses a newspaper story which suggested that al-Qa'ida had taken over the HSU and that the prisoners have been radicalised.

Misbehaviour in the HSU is dealt with by punishment in the form of the segregation unit, where prisoners must spend 23 hours a day in their cell. And if an inmate is particularly troublesome, he will go in "the box": a room with nothing inside it except a perspex window.

Due to its added security and smaller prisoner numbers, the safety record of the HSU is better than the main prison. There has been one accidental death – where a bag of drugs burst inside a prisoner – and one incident where an IRA prisoner attempted to slit his throat.

But, unlike the main prison, where there have been three suicides in the past three months, no one has ever killed themselves in the HSU.

And, despite the differing crimes of the men in the HSU, Murray says that the atmosphere is generally good. "Prisoners over here have to get on because it is such a closed environment," he said. "You would be surprised at who gets on with who. There are no gang or religious affiliations. Muslim prisoners and non-Muslim prisoners get on very well.

"And from my experience prisoners going from here back into the main jail hate it. In the main jail, prisoners do not have the same amount of contact with the staff. Also for many of them it gives them kudos to be over here."

While the prison staff are careful not to discuss the identities of current inmates, they enjoy name-dropping previous prisoners: "I remember when I heard Charlie Bronson banging on his cell door..." and, "When I first met Ian Huntley..." are the beginnings of of two tales I heard.

And they are honest enough to admit that even they are intrigued by the men they look after. Senior officer Jason Hancock explained: "There are evenings when I will be watching the news and hear about a load of terrorist arrests and, because of the type of prisoner we look after, I think to myself: 'I'll be seeing them in the morning'.

"And I have been known to go home and tell my wife: 'Guess who I bumped into today...' But there are many shocking things that we see that we do not go home and tell our friends and families about.

"A lot of things stay within the prison and the officers have their own coping mechanisms. Some of us tell jokes about things because we don't like to admit that something has affected us more than we let on."

One of the most intriguing things about HMP Belmarsh is the unique "two-prison" set-up. Outside the HSU is a local prison with convicts on short-term sentences. Not only are the prisoners separated, but the guards' paths do not cross either. And in some ways the conditions in the main jail are worse than those in the HSU.

John Steadman, a 40-year-old convicted cocaine dealer, is 15 months into a five-year sentence. He is sitting in his cell watching television when I visit.

"Prison is boring and repetitive," he says. "This is my first sentence and definitely my last. Yes we get to watch television and play pool, but those things are just something to kill the time with. You could put a sauna and a sunbed in here and I'd still rather be outside."

It is not just the prisoners who have complaints; the guards often mention the low levels of staffing – there are just over 400 officers on rotating shifts and they are acutely aware that they are always vastly outnumbered by the prisoners. They also have to deal with drugs and mobile phones being smuggled into the prison. This is particularly annoying due to the fact that many are brought in by corrupt guards.

Despite this, Phil Wragg, the governor, is happy with his prison. "This is the best command in the prison service," he tells me. "We have the highest security and the most resources. It is also the most expensive prison to run.

"Yes, it has a bad name, but we do a good job. It has a bad name for all the wrong reasons. It has a bad name because people write things about it who, frankly, are not qualified to do so. And it is certainly not a holiday camp."

It doesn't look like one either. And obviously the prisoners inside agree. As I leave the prison I walk past guards with dogs and an exercise yard full of prisoners who press their faces against the wire fences. One shouts out to me. "Let me tell you lad," he says, nodding towards the gate and the outside world, "You are a very lucky man."

Belmarsh's most notorious prisoners

Abu Hamza

Extremist cleric who lost his hands in an explosion. On remand pending extradition request from the US.

Waheed Zaman

A relatively new arrival to Belmarsh, he was convicted of plotting to bomb a transatlantic flight.

Curtis Warren

One of Britain's most dangerous gangsters, Warren is currently serving 13 years for smuggling drugs.

Charles Bronson

Dubbed Britain's most violent prisoner, Bronson once had an entire wing of Belmarsh HSU to himself.

Ian Huntley

The Soham murderer was held in Belmarsh's high security unit before his trial and conviction in 2005.

Kenneth Noye

Road rage killer who fled to Spain after stabbing Stephen Cameron, 21, in Swanley, Kent, in 1996.

Ronnie Biggs

Spent 36 years on the run and then eight in Belmarsh. Released last summer on compassionate grounds.