Masud, 15, died on a lorry after losing hope that his claim to live with his sister in Britain would ever be heard

Just before the New Year, Masud decided he couldn’t wait any longer. Christmas had come and gone. Life in the sprawling Calais “Jungle”, a squalid home for up to 6,000 migrants, was becoming increasingly unbearable for the 15-year-old. He had a legitimate claim for entry to the UK because he has a sister living here. But no one, least of all the British government, was listening.

Mohammad Nabi, a fellow Afghan who had shared a tent with Masud since the teenager arrived alone at the Jungle three months ago, said: “He had extremely high hopes for the future and was looking forward to a life in Britain. But as he waited for his case to be heard he thought: ‘Well, why not try myself?’”

On 27 December, Masud, who had travelled from the Afghan city of Mazar-e-Sharif, where the Afghan army was battling militants only last week, packed some belongings and decided to take matters into his own hands. He headed to nearby Dunkirk and, the following day, managed to climb into the back of a lorry headed for England. “After a few days we wondered what had happened to him,” said Nabi. “We began calling but his phone rang out, then we started texting: “Where are you? Are you OK?”

Finally, on 3 January, Nabi received a text from Masud’s phone. The lorry carrying the teenager had been pulled over by police outside Dunkirk ferry port. Inside the authorities made a grisly discovery. The young Afghan had suffocated and died.

The tragedy – the first known refugee death in northern France this year – has prompted anguish over the government’s approach towards unaccompanied children in Calais. And the pain of those who knew Masud is deepened by the knowledge that he was about to become a test case. He had left Sweden for Calais in September in an attempt to be reunited with his sister. That meant he had a right to enter Britain.

Under a clause in the EU’s Dublin Regulation covering hearing of asylum claims, refugees who have close family members in a particular EU country can claim asylum there. But according to campaigners, the Home Office has in numerous cases chosen to ignore that right. Masud was among a group of children listed in a legal challenge against the Home Office that will be heard in London on 18 January. Lawyers had handpicked the young Afghan as one of the most desperate cases in Calais – a vulnerable and lonely child who deserved to be urgently reunited with a family member.

“Masud was a 15-year-old boy in need of protection, a boy in need of his sister here in the UK,” said the Bishop of Barking, Peter Hill, a spokesperson for campaign group Citizens UK. “Every single night, desperate children are climbing into lorries and jumping on to train tracks to try and reach their families. Our government must act to honour its obligations and help these children.”

On Saturday, Citizens UK launched an online petition demanding that David Cameron act to stop more teenagers from dying as they attempt to reach their relatives. Shadow immigration minister Keir Starmer, who visited the Jungle on Friday, announced that he would be writing to the home secretary, Theresa May, on Monday, asking why pledges she made last year to implement the Dublin Regulation and help the most vulnerable migrants – unaccompanied children – had led to so little action on the ground.

Aid charities have reported a surge in the volume of unaccompanied youngsters living in tents in Calais with no support from the French state. Others warn they are vulnerable to traffickers, describing the child protection issues as enormous. “We are aware that there is an increasing trend for unaccompanied minors to be facilitated into and across the EU,” a Europol spokesman said, adding that about 7,000 had been reported among the flow of refugees – a figure they said was rising.

Liz Clegg, who runs an independent women’s and children’s hospital in the Jungle, estimates that there are hundreds of unaccompanied children in the camp at any given point. An estimated 1,600 were registered by child protection agencies in France during 2015, although the true figure is certainly greater because most unaccompanied minors avoid the authorities.

To date there has been no attempt to count the actual numbers of children gathered in northern France, although a French NGO, Trajectoires, will begin a census this month. Last Tuesday, a committee of MPs urged the UK to resettle 3,000 unaccompanied children from Syria, in addition to the 20,000 refugees it has already agreed to accept.

Facebook Twitter Pinterest The young or unaccompanied frequently find life in the camps difficult. Photograph: Carl Court/Getty Images

Provisional work to identify Jungle migrants who have a clear legal right to live in Britain under the Dublin Regulation has tentatively identified 200, scores of them unaccompanied children.

“They all have the right to transfer their claim to Britain, so what is going on? It’s disgusting,” said Clegg. Charities warn that many unaccompanied minors simply disappear from the Jungle, often mysteriously. “We’ve even had siblings who don’t know where the other has gone,” added Clegg. “Sometimes we never hear of them again.”

Of the seven refugees who once shared a tent with Masud and Nabi, four are believed to have made it to England, one is in Paris, the youngest is dead and Nabi remains in the Jungle.

Elizabeth Fraser, whose charity Miracle Street provides a generator for refugees to use in the Jungle, said: “So many unaccompanied children seem to go missing. I remember once there were three Afghan brothers aged 10, 13, 16, and after a few days they also went. Nobody knows where to.”

Sixteen-year-old Tarek, from Syria, was last seen walking from the Jungle towards the port of Calais on 28 December and has not been seen since. Fraser has tried calling to no avail, although she knows that the phones her charity dispenses to the young are frequently stolen by older men in the Jungle, or confiscated by police.

Videos of Tarek shot by Fraser shortly before the teenager went missing show him struggling to cope with his situation: at one point he sobs uncontrollably when his family are mentioned.

Unaccompanied children are particularly vulnerable to the French police’s routinely aggressive approach to the Calais refugees. Every unaccompanied child the Observer spoke to testified to alleged police brutality. All claimed to have had either pepper sprays, water cannon or truncheons used on them. Khalid al-Bai’aa, 16, from Egypt, described a bloody head wound caused by a blow from police. Hossam, 13, from Alexandria in Egypt, who has been in the Jungle since September, said: “I’ve been pepper sprayed in the face which is very painful: the police cause me many problems.” His friend Mahmoud, 17, also from Alexandria, whom he met in Calais, showed a blemish on his eyelid that he claimed was caused by pepper spray.

Within the camp, the young and unaccompanied find themselves at the bottom of a hierarchy of bullying from groups of older males. Bai’aa described waking to find his tent slashed open and his rucksack and clothes stolen. On another occasion he was mugged.

Intimidation is rife, said Mahmoud. “Even in the lines queuing for food groups say bad things at me and swear. Once they scratched my face.”

There are other challenges, too. Bai’aa has been constantly ill since arriving four months ago; throughout the interview he clutched a bottle of chest medicine.

“It’s cold and wet and health is a major issue. We’ve had a few in hospital with pneumonia,” said Clegg. Fraser added: “The cold gets into your bones and there’s no way to warm up. The faces of the children are turning grey. Their eyes are red and sunken.” Many complain they barely get enough to eat.

Yet their desperation to get to England remains unquenchable. Hossam makes an attempt to climb on board a truck most days, and once spent 24 hours lying motionless inside a lorry before the police found him. Tonight he’ll go again, risking the same fate as Masud.