An Afghan and I once broke into a small house in Peckham. I had met him while researching a report on immigration for the Daily Mail.

We were on a rescue mission. Inside the two-up, two-down property in South East London, every room was packed with beds, mattresses and migrants.

On the landing, grey and scarcely breathing, lay the man we had come to rescue from this awful place.

Labour has let London turn into a foreign city where even tramps are immigrants, writes Harriet Sergeant

The other migrants who were there started to shout at us. Apparently, the sick man owed them and their trafficking gang money. We had to hustle him out fast.

I emerged shaken. I had glimpsed a London I did not know existed — one of Third World poverty, exploitation and criminality.

It is writer Ben Judah’s great achievement to reveal that hidden city in his new book, This Is London: Life And Death In The World City.

A young war correspondent, Judah examines his home city as the foreign metropolis it has now become

Since 2001, immigration has transformed the capital. More than half of Londoners are now not ethnically British.

As he says: ‘I was born in London, but I no longer recognise this city. I don’t know if I love the new London, or if it frightens me: a city where at least 55 per cent of people are not white British, nearly 40 per cent were born abroad and hundreds of thousands are living illegally, in the shadows.’

Who are these new Londoners? In order to find out, Judah immerses himself in the migrant world.

He spends a night with the Roma beggars who camp in the tunnels beneath Hyde Park. He stays in a doss-house in Barking, East London. He cajoles an astonishing array of migrants from across London to describe their lives.

Whether a mini cab driver who also washes dead bodies, a Romanian prostitute in a blonde wig, or a wealthy young African with bodyguards hired from his own tribe, they open up to Judah.

The quantity and authenticity of his interviews build up an irrefutable argument. Here, as opposed to what the Left is always telling us, are the real effects of immigration.

Immigration, Judah makes clear, has touched every aspect of life in London and utterly transformed it.

The English upper classes no longer inhabit the splendid townhouses in Mayfair. Suburbs such as Edmonton in the north of the city are no longer home to the aspirational, largely unionised, English working-class.

As for white, East End gangsters such as the Kray Twins, you will only find those in movies.

The same goes for the prostitutes who used to inhabit Soho. Even tramps, for goodness sake, are rarely English any more. Such is the level of liberal propaganda that we have largely remained blind to this startling transformation of our city.

Judah systematically maps these changes — the result of Labour relaxing immigration into the UK.

Left-wingers and the business establishment pat themselves on the back for creating an open city that welcomes the world. In fact, we have lost control of our borders and have no say who comes here.

This week, it emerged that Brussels is attempting to blackmail Britain by saying that if we don’t take as many as 90,000 migrants a year, we will not be able to send failed asylum seekers back to safe countries on the Continent.

Then came the story that a tribunal had ruled that a group of migrants in the Jungle camp in Calais had a ‘human right’ to join family members in Britain. It is a precedent that could have far-reaching implications.

Aside from legal rulings, the truth is that much of our immigration policy is now dictated by criminal trafficking gangs who make a fortune smuggling people into Britain — not our elected leaders and certainly not voters and taxpayers.

It is criminal gangs who decide who comes into the UK and in what numbers. They largely decide what happens to the migrants once they arrive.

In handing our immigration system over to them, we have allowed conditions of unimaginable squalor, misery and criminality reminiscent of Victorian times to take hold.

So how has it happened? Well, get rid of any notion that we are doing the world’s poorest many favours.

Judah quickly discovers that multiculturalism for many migrants is a euphemism for slave labour.

It starts when would-be immigrants listen to the sales pitch of the agents for the people-traffickers. They promise that London is ‘a second paradise’ where ‘every man is rich’.

On offer is free NHS treatment, free housing, free schooling and countless welfare benefits. It is a place of opportunity, security and, above all, available women.

This was a definite pull for one young Afghan working in a butcher’s in Neasden, North West London.

As he put it, in his village, if you slept with more than one person in your life, ‘they shot you in the back’. In Britain, however, sexual opportunities were immense.

Homeless Roma immigrants, pictured, can now be found sleeping in doorways in Mayfair

The agents explain how to take full advantage of the UK.‘London is a country of rights,’ they say. How very true! If you tell the correct story, they are informed, ‘you will never be sent home’.

And yet, when migrants manage to reach the UK, they all have to pay back the people-traffickers who got them here.

Indeed, it is this debt, sometimes with interest rates of 100 per cent, that lies behind the often appalling working conditions they are forced to accept.

This is the simple fact that the immigration lobby ignore — with appalling consequences for the migrants concerned.

Typical of the views of those Judah met was someone who said: ‘I thought the money would be growing on trees. Six months later, I was crying myself to sleep. I was homeless.’

A Roma violin player in a tunnel beneath Park Lane explains what this means.

‘We’re all here to beg, to work off our debts. We give the enforcers all the money. They told us to come here. But we are never going to make back the loans they gave us to come here. We’re trapped.’

He explains that the people-traffickers are a constant threat to the children migrants have left behind in places such as the Romanian city of Slobozia. Defiance could bring brutal retribution on those they love.

Judah quickly learns that the people who make it here are ‘virtual slaves’. And it is the same wherever he goes. In a hotel laundry room, Africans curse the tricksters who brought them here on counterfeited visas and passports.

A Ghanaian came on a student visa with plans to set up an international business. All he wanted, he said, was to wear a suit and work in an office.

Yet the traffickers never told him what would happen when he tried to work as an illegal migrant.

He used up his savings of five years in a matter of months. Instead of running his own business, he stacks shelves in a warehouse.

Frustrated, angry and exhausted, he knows he has ruined his life. He can never escape, never get home to his children, because his low wages mean he cannot imagine ever paying off his debt.

There is a whole illegal city in London of several hundred thousand people, and nearly half of them are thought to have arrived after 2001.

This is a city hidden from official statistics, but not hidden from employers who take on the workers, no questions asked, through agencies.

Legal workers also have problems because the sheer number of new arrivals push down wages.

A Polish builder explains that, on his work site, the English builders tell him angrily they used to be paid £15 an hour. Now it is £7 an hour. ‘They hate me,’ he says simply.

But then he gripes about the newly-arrived Romanians who, he says, are pushing wages even lower.

Tragically, politicians appear blissfully oblivious to the new realities. They solemnly debate the merits of a ‘living wage’ over the minimum wage.

What planet are they living on?

Certainly, it’s one as far removed as possible from the reality of daily life outside Wickes, the builder’s merchant in Barking, East London.

Here, Judah joined 80 or so Romanians, ‘blank eyes wide with hunger’, touting for work.

‘They pace up and down in their work boots for the next 11 hours, muttering tensely, ‘waiting for a white van’.

This is a reference to the fact, they tell Judah, that the proverbial White Van Englishman is the best type of employer. Whereas Pakistanis, Turks and Poles are the worst.

One explains: ‘They make us fight for work. And they know we have no choice. That we are hungry.’

To such men, the minimum wage is a luxury that they can only dream about. They often work for much less — or, as one put it, for nothing more than ‘one chicken and chips’.

But this story is not just about brutal exploitation. It is also about crime.

Along with immigrants, we have imported the criminal gangs who blight their countries. When it is so easy for migrants to get here, it is just as simple for the world’s nastiest criminals to relocate to the UK, too.

Thus many Somalian gangs terrorise estates in South London and Turkish gangs control North London.

Kurds and Albanians launder their money through the car washes in Tottenham and Kilburn. Vietnamese gangs grow and distribute two-thirds of the strong cannabis on the capital’s streets.

The EU in Brussels has told David Cameron the UK must accept 90,000 migrants per year

Judah encountered a Grenadian, now a successful cocaine dealer, who recalled moving to the White City estate in West London with his mother when he was 12.

‘We pitched up for a better life, but found ourselves right in the middle of a war zone,’ he recalled.

He said the estate ‘was way more corrupt . . . way more dangerous, more full of disillusion than anywhere in Grenada. Within six months of being here, I had lost 75 per cent of my morals.’

After a few years here, and now a heavy cocaine-user, he woke up to his mother screaming. She’d found his gun in the fridge and bullets on the sofa.

She was crying: ‘Please, please, the police will kill you, the gangsters will kill you! My baby, why, why did I ever bring you to this country?’

The sex industry is another area where our flawed immigration policy has had a malign effect. Now, 96 per cent of prostitutes are migrants. In the main, Albanians have taken over.

Typically, they lure girls from Moldova with promises of modelling jobs, but then rape and traffic them.

I saw the results of this myself when a brothel opened in a house on a quiet, residential street near my son’s primary school in Hampstead.

The mother of one of my son’s friends lived opposite. She was intimidated by the sinister men in leather jackets who sat in the nearby coffee shop all day.

She reported to the police that the girls on the top floor looked underage and never went out. In due course, her car was smashed up.

She suspected this was done as a warning to keep her nose out of it. The police did nothing and she never raised the matter again.

Finally, migration has even changed the nationality of London’s vagrants.

There are now around 5,000 — mostly Polish and Romanians — living rough on London’s streets. In North London, they unload trucks for Turkish shopkeepers in exchange for nothing more than their drink of choice, White Ace cider.

Others have been found roasting rats for food in the back alleys of Tottenham and Haringey.

In this new London, increasingly there is one nationality significant for its absence — white, British-born. Nearly all the migrants comment on this phenomenon.

Ben Judah meets a Pole who works as a registrar — recording births and deaths — in Catford Bridge, who says she works in a position that is perfect to spot the ethnic changes.

She describes the new London where 57 per cent of births are to migrant mothers.

But when she enters the names of the recent dead, they are nearly all of them old white British. Another migrant comments: ‘The English are dying. They are declining fast.’

He recalls in the street markets, there used to be only English voices shouting out, ‘advertising their wares in the Cockney accent. But they’ve gone now’.

A Met policeman, who was born in Nigeria, says: ‘The English are vanishing. London is no longer an English city at all . . . London is a patchwork of ghettos.’

Between 1971 and 2011, the white British share of London’s population slumped from 86 per cent to 45 per cent, overwhelmingly the old Cockney working-class. Cockney, in fact, is predicted to die out in 15 years.

I once visited a school in the East End displaying photos of a class from the Thirties and today. In the first photo, the children were all white. In the second, none of them were.

In some countries, this might be called racial cleansing. In London, it is simply the result of the failed policy of multiculturalism.

English is no longer the predominant language heard at London street markets, pictured, any more

On the Old Kent Road, Judah found only one English shopkeeper left — a plumbers’ merchant. He charts the gradual disappearance of the English by the fate of their pubs.

Along this road, there used to be 12: one has been turned into a Nigerian mosque, another into the Afrikiko nightclub, the rest have simply been demolished.

Across London, it is the same story. African churches move into abandoned bingo halls, cinemas and pubs vacated by the old white working-class.

In Brent and Harlesden, the white population has fallen by 30 per cent since 2001.

Suburbs such as Edmonton, once ‘terraces of white respectability’, where the Tory politician Norman Tebbit grew up for goodness’ sake, have now turned into tenements for migrants.

This huge demographic shift is as true for wealthy areas as for deprived.

Half of Kensington & Chelsea’s inhabitants were born overseas — a third arriving since 2001. Forty per cent do not have a British passport.

Global immigration has transformed these areas as radically as it has other parts of London.

The English upper and upper-middle classes find themselves elbowed out of Mayfair, Knightsbridge and Notting Hill Gate — as surely as the white working class has been out of London’s East End.

So how can the relatively honest, tax-paying English compete to buy property with a global elite who, as one wealthy young Nigerian explains, are not paying taxes, and are making their money in countries rife with corruption?

Instead, the next generation of these English retreat to places such as Peckham — as Judah put it, ‘mourning their postcodes’, and ‘bitter like refugees’.

Judah has done an extraordinary job in painstakingly building up a vivid and desperate portrait of London today.

His observations turn an unsparing spotlight on how the British people have been so befuddled by Left-wing dogma and political correctness that too few people have noticed the utter transformation of our capital city.

Too many have failed to question why we lost control of our borders and, having done so, allowed immigrant gangs to exploit migrants.

Also, we have failed to wonder if we are comfortable with this new London, with its extremes of foreign poverty and wealth.

Uncontrolled immigration has changed London for ever. And, most shamefully, not one of our leaders has lifted a finger to stop it.