We're the only broadcast journalists given access to one of Britain's most dangerous jails since the current crisis.

And some prison officers are so frightened of being targeted on the outside that we've had to blur their faces.

Staff are overstretched and on the verge of breaking point.

You worry about, you know, opening the door in the morning and making sure that they're alive.

Anything can happen.

There's been deaths in prison.

Alright, what have we got coming this afternoon then?

Each day more busloads arrive, making this jail one of the most crowded in the country.

Can you stand in the middle of the black and yellow box for me?

They undergo body checks.

So this scanner will show whether they're hiding anything inside their body, any drugs, any weapons.

Just there, there's a gap, that'll be the entrance of his anus basically.

Gosh, you would think that would hurt.

As of tonight there are just nine spare beds.

Most of the inmates are on remand, so they're waiting for their trial or sentencing.

Others are doing time for crimes such as murder, robbery and rape.

And anything can happen.

There's an alarm going off.

It's a code red.

A code red alarm involves blood or burns.

An inmate has self-harmed.

A moment of apprehension as they assess the situation.

Can you talk me through what's happened?

Yes, we've got a prisoner who's self-harmed here.

He's made superficial cuts to his arm.

We were told he'd carved the words mum and dad onto his skin with a sharp object.

Staff say overcrowding is leading to an increase in self-harm and violence because of inmates' frustration with being locked up for longer.

Across the wing, Michael's inside for drug offences.

There are some people who simply don't care about prisoners.

They're like, they broke the law.

They should be locked up.

Shouldn't be a hotel.

But you are supposed to be rehabilitated inside.

Do you think prison is providing a rehabilitation?

No.

It's a punishment.

It's not a rehabilitation.

It's hard to rehabilitate yourself in a place where you've got gang violence, postcode wars, drug violence, money wars.

If you've got beef there, violence there, drugs there, people collapsing here, the staff, they're outstretched.

They're trying to do this, this, this and this, but no, you want help as well.

Moments later, another alarm.

The fifth of the day.

Yesterday, there were more than 30.

So they've just jumped on the netting.

Prison officers are running up the stairs to try and get them down.

What the staff are saying is that it can turn really quickly.

A quiet moment can become chaotic within a matter of seconds.

As one prison officer said, it's like the flick of a switch.

Staff do what they can to calm things down.

And very quickly, they're back on normal duties.

They all kind of blur into one because it's not like, oh, once in a blue moon, something happens.

It's like day in, day out.

You kind of are dealing with incidents from self-harm to violence to kind of people with severe mental health issues.

I've had damp in here for three weeks.

And then there are complaints about the cells.

It doesn't smell good in here.

What's that?

The toilet's been leaking.

Look at the toilet.

Come and have a look at this one.

Oh, my word.

I've been telling them about that for three weeks.

Gang violence is an ongoing threat at Pentonville and has had some serious consequences for staff.

This prison officer broke both her wrists while separating two gang members during a fight.

It's probably the biggest thing in London.

Like, the gang affiliation, gang issues are the biggest reasons why we have so many probably people in jail.

So it ends up us trying to stop them, just trying to stop the fight.

That's how I broke both my wrists, just trying to stop it.

That wasn't even, yeah, it just gets really messy sometimes.

And they're quite small space.

Stressful?

Stressful.

Yeah, stressful.

Yeah.

No, I'm shaming you.

Mark is waiting to be sentenced and has been here for nine months.

When they come into here, you don't know who's on this wing.

Every other wing has got allocated for every gang member.

What's the worst case scenario in your mind?

Fights, killings, all sorts could happen.

You don't know.

Like, when they're such in close proximity, then it does blow up.