I was interviewing ten foster parents in west London for a report on children in care. Foster parents are in great demand, so I was startled to discover that only one of the sets of parents was looking after the sort of vulnerable children you imagine to be in the care system. The others were looking after unaccompanied asylum-seeker children.

They made an alarming claim: three of these seemed to be adults passing themselves off as boys. ‘The first thing they ask for is a razor,’ said one foster parent, ‘They’ve got these big beards.’ A woman admitted she found it embarrassing having a grown man posing as a 17-year-old. But the authorities appeared uninterested. ‘Our concerns are just fobbed off,’ said another.

A counter-extremism expert told me: ‘There is nothing in the system to stop a 26-year-old Isis fighter coming here, stating he is 17 and claiming asylum.’

Anyone forced to flee his or her country with a well-founded fear of persecution can claim asylum. An orphan under 18 has special rights. They receive the same benefits as a child taken into care. No one would begrudge a genuine child refugee these privileges. The problem is the system is open to abuse, and the latest terrorist attack in Parsons Green raises further questions. Ahmed Hassan is an 18-year-old unaccompanied asylum seeker who is alleged to have built the bomb in his foster parents’ kitchen.

We do not know how he came here or what could have led him to do what he is accused of. But it is time, surely, to question our asylum system for refugee children. Yet raise concerns and you risk Gary Lineker labelling you ‘hideously racist and utterly heartless’.

The problem is sorting myth from fact. The first myth, emphasised over and over again, is that these are vulnerable children. The word conjures up images of small boys and girls. Our hearts break for them. The reality is somewhat different.

Only 8 per cent of unaccompanied minors who arrived in the UK in 2015 were, in fact, under 14. This is according to Eurostat, the statistical office of the European Union. Instead, over half were aged 16-17. Nor was there a balance of boys and girls. Some 91 per cent were male. Save the Children admits, ‘Many come across as being self-reliant and not in need of support’, but it urges that they are often ‘extremely vulnerable and in need of reassurance and care’.

Why are the majority of refugee children in fact teenage boys? Here we come up against another myth: that refugees somehow find their own way to the UK. The truth is, nobody arrives in this country without the help of a people trafficker. This means it is the people traffickers who control our immigration system — not the Home Office. It is they who dictate who comes here. Refugees who cannot afford to pay never make it. If we really want to help the vulnerable, we should be taking children directly from refugee camps.

The central role of people traffickers means that every young person arriving here represents a considerable investment by their family or community back home. This explains why they are nearly all young men. They come from cultures in which men have greater earning power.

As one immigration officer at a busy UK airport with 20 years’ experience of dealing with refugee children explained to me: ‘Ninety per cent of them are not orphans. Their coming here is very well worked out. Their families have paid the people traffickers to bring them here. The intention is for the families to follow shortly after. These are cash-rich young people.’ For the most part, in his opinion, ‘They are not fleeing for their lives.’ In other words, they are economic migrants and therefore not entitled to asylum.

His view is backed up by the actions of the traffickers themselves. As Save the Children warns, the agent will instruct the young person to lie about their nationality and age and destroy all identity documents. Some gangs even provide a pack with information on claiming asylum, fake documents, a ready-made asylum story and a pair of scissors. The only reason for doing this is because they know their clients are economic migrants.

The third myth is that every child refugee is speaking the truth. Actually we have no way of knowing. The lack of documentation means that the most basic facts about a young person cannot be checked: their age, for example, their nationality or even their real name. When I sat in on interviews of adult asylum seekers by determining officers, I was amazed at the vagueness. One man could not remember how long he had been in prison: ‘Maybe one month, maybe one year, maybe many more.’ The tactic is a deliberate ploy promoted by the gangs, whom asylum seekers fear more than the Immigration and Naturalisation Service.

The immigration officer explained: ‘For years now we have had adult Pakistani males arriving in this country maintaining they are Afghan teenagers. They tell me they are 13 or 14, but they are clearly over 20, well developed and with good facial hair.’ In 2015 the second largest number of claimants came from Afghanistan.

The co-operation between traffickers and extremists is a new and alarming threat to our national security, points out Rosalind Ereira of Solidarity with Refugees. Some migrants sign up to support Isis in exchange for their travel; the money paid by others to the smugglers ‘helps fund Islamic State activities’. A report from Quilliam, a leading counter-extremism think-tank, warns: ‘There is no question that militant groups target refugee youth for recruitment.’

The immigration officer is frustrated because he knows by sight many of the ‘facilitators’ or people traffickers. These are often young men on benefits who appear mysteriously able to travel ten times a year to Dubai and Africa. They charge a high price for a personalised service in which they accompany the young migrants on the plane before leaving them at the terminal. But the traffickers have British or EU passports. ‘I have no power to stop a British citizen longer than five minutes otherwise my bosses upstairs will kick off. I can do nothing without the traffickers’ permission. Nothing — and they know that.’

Despite the security threat, few in authority appear willing to tackle the problem. When a Conservative MP suggested checking the age of young asylum seekers with dental or X-ray tests of the hand to measure bone density, he was accused of ‘vilifying’ refugees. Ruth Allen, chief executive of the British Association of Social Workers, said medical tests would be ‘very intrusive and could be re-traumatising’.

When Norway insisted on a dental examination of arriving refugee children, they discovered nine out of ten were, in fact, over 18.

As a social worker, Allen must know the dangers of introducing grown men into schools and foster families. Paul Chadwick of Croydon borough council warned a House of Lords Committee last year of sexual exploitation in schools ‘by adults claiming to be children and placed in a school’. A worker in a residential home in Kent for children in care said that half of the children there are unaccompanied asylum-seeking children. In her estimation, more than half the migrants are not children at all, but in their twenties.

‘They can be quite frightening at times,’ she said. ‘They are aggressive and have an attitude problem. Many have no respect for women because of their culture. No one is giving consideration to the risks they pose, not just to staff but to the other children in the home. Because they are older, they have a lot of influence on the youngsters, who are very vulnerable. They introduce the children to alcohol and get them into crime like street robberies. It is a serious problem, which those in authority are not tackling.’

There is another issue. Our most vulnerable children are in competition with these asylum-seeking young people for a limited number of foster parents, a limited number of places in care homes and, above all, a limited amount of money.

Explaining who is losing out, a social worker says the system has ‘moved away’ from providing a service to the British kids in its care: ‘Instead we are dealing with problems particular to young asylum seekers — their legal status, visits to the Home Office and so on.’ She went on angrily: ‘This at the expense of our own 16- to 17-year-old care leavers who need a lot of support and are not getting it.’

The investment made by their families means the majority of the young migrants are, as the heads of various social services confirmed, ‘very motivated, see it as an opportunity and do very well. They are largely middle-class, male and expect to go to university,’ said one.

What a contrast to the care leavers I interviewed. At the age of ten, Trevon came home to find his crack-addict mother hanging dead in the kitchen. The lives of these kids are desperate. But Lily Allen does not cry for them on camera and it is almost impossible to get them the help they need.

It is time to overhaul a system that is corrupt, dangerous and fails to help the most deserving. But don’t hold your breath that anything will change, despite a vulnerable child trying to blow us up. The immigration officer summed up the general frustration: ‘You try and apply the rules only to be hauled up from on high and told “to deal with it”. I get bitter and twisted about it,’ he said. ‘We are heading into desperate times.’

Harriet Sergeant and Andy Elvin, CEO of fostering charity TACT, on the Spectator Podcast.