Alexander has been threatened at knifepoint four times in the streets that surround his home in East London, so he has learnt to live by a system “so you don't get penalised”. This means never looking at a stranger for too long and never more than once, because “they might shank you” - street parlance for being stabbed.

It means, among other things, wearing two pairs of trousers. He places his valuables in the obscured pair so that he can fool potential foes into thinking that he has nothing worth stealing. Confrontation can easily escalate into violence.

Alexander has thought about carrying a blade for protection, but he dismissed it because “it's not worth getting caught”. He has a tag name, Korrupt Kid, but admits that he made it up only because all his friends had one.

An air of solemnity comes over him when he is asked to consider his exposure to knife crime: “It's quite scary to think that anything can happen to you at any time. I get angry when I have to think about if I'm going to get home safely, and sometimes I think that I won't. But it comes naturally to us. Mum says it never used to be this dangerous. I think I must have been born too late.”

Alexander is 12 years old.

Ten miles away in North London, Emma Parker, 16, admits that she fears for her safety and, although she has never carried a knife, she knows plenty of people who do. “Boys and girls, sure. They have to carry something, it's always for protection.”

In Wood Green, Jay, 14, appears cheerful but he is betrayed by his darting, nervous eyes. He admits that he is “a bit scared right now”, walking through the postcode N22, because he is from Archway and hangs with an N19 crew, Busy Blockz.

“Shouldn't be here. My cousin, he's in MOB (Money Over Bitches). This is their area and if he sees me, he'll beat me up.”

Her arms folded and rolling her eyes, his girlfriend Zara, 15, admonishes Jay for belonging to a “stupid gang”.

“It's all about upping your rep. You wanna look cool, you don't want to look like a little boy. What street cred? I worry he's going to get hurt.”

For these youths, and many others, theirs is a problem that transcends age, race, gender and postcodes. If it was unclear before, a recent spate of stabbings has confirmed that the lives of a significant proportion of the capital's adolescents are being shaped by exposure to violence.

This week The Times spoke to scores of children across the city. Stories of violence, and of dead and injured peers, spilt from their mouths as fast as the street slang.

They talk of getting shanked or opened up (stabbed), of getting burst or popped (shot). Giving someone the screwface is staring at a rival in the prelude to a fight; hotting someone up is antagonising them in the hope that things will escalate. Jacked, rapped, clamped and eaten - all refer to belongings being stolen.

Many teenagers are not involved in any trouble but know how to get their hands on knives. Natz, 15, looked incredulous when asked about obtaining knives, because apparently it is obvious: “You just go to the shop and say it's for constructive use, or you import the big ones from Italy. Everyone knows.”

On the street, the practice known as “repping your end” or “stating your noize” is blamed for a lot of the stabbings.

“It's every area against every area,” says Jerrell White, 17, from Lambeth, South London. “If you're from Peckham you hate every other area. If you're from Clapham you hate every other area. It's stupid, but some people get sucked into it. So if someone comes into your area, you get them.”

In the north of London, that comes down to Edmonton v Wood Green v Tottenham v Hackney. In the south of the capital, Clapham v Brixton v Lewisham v Stockwell v Peckham v Wandsworth. Youths are asked to “rep your end” - state your postcode. They learn how to run fast if they give the wrong answer.

Tony, 15, from Peckham, visited his grandmother in Brixton this week and, when asked, made the mistake of telling a group of 20 youths that his postcode was SE15. They chased him for about 20 minutes - one had a knife, the others had bottles.

“If they caught me, they'd probably have bottled me, stabbed me, kicked me, punched me,” he said. “Doubt they'd have killed me, not on purpose, though maybe in the process.”

He shrugs: “Happens all the time.”

Some blame gangsta rap and “grime” - a form of urban music developed from hip hop and garage - for fuelling what politicians are labelling a crisis.

A story was spreading around East End teenagers this week about two gangsta rappers from rival suburbs. One is said to have shown up at the home of his adversary's mother, placing a gun to her head and ordering her to make him a sandwich.

“It was an insult, you see?” says 15-year-old Matthew, who lives in E12. “He was saying how small the other guy was to him. If he was proper angry with him, and he cared, he would have shot her in the head.”

Matthew, who had earlier told how he was stabbed in the leg when he refused to hand over his phone to a youth, is unable to finish his story properly. No, he does not know whether it was a chicken or tuna sandwich. His friends roar with laughter.

It may be true, it probably isn't. But it is the blasé nature with which Matthew - and so many others - tell stories of such violence that makes their situation so shocking.

Tony, the boy from Peckham, flippantly points at his chest, arm and leg before explaining that he has been stabbed four times.

Once again, he shrugs: “It happens. They knew the area I was from and so they got me.”

Paul Kassman, a consultant on projects for inner-city youth, says that in this kind of territoriality, the “smallest cross word or the smallest type of challenge” will trigger a retaliation.

“Their sense of identity is weak. They get a particular estate or area or neighbourhood and they form a group. If a kid gets into conflict with another area, it becomes a group issue. They feed into a super-macho and super-violent ethos so things escalate very quickly.”

Charities across the city are working with troubled teenagers in an attempt to steer them away from violence. Boris Johnson, the Mayor of London, plans to increase the police presence on public transport and the Metropolitan Police announced a stop-and-search drive in response to the growing number of teenager killings.

But the youth in London's inner-city suburbs are wary of police.

Paul Ayul, 19, from Southwark, said that the police did not realise how easily they disenfranchise youth, particularly black youths.

“They say that one in five people they catch is doing something, but what about the other four people that don't have nothing on them? It's the majority that are innocent that are getting accused. There's a lot of anger out there.”

The youths agree that while they are not immune to the violence surrounding them, many have become desensitised by it.

Jay, from Wood Green, knew Martin Dinnegan, who was stabbed to death last year at the age of 14. Another of his friends was stabbed in the park last week.

It is almost an afterthought, 15 minutes later, that he remembers that another friend was stabbed a couple of weeks ago. “That's right, forgot about that. It's sad, but it happens all the time. Nothing you can do about it. Just got to live with it.”

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