More than a year after the UK government pledged to transfer hundreds of child refugees from Greece, the first unaccompanied minor from the country will arrive in London this week.

However the 15-year-old Syrian is described by experts as profoundly traumatised because of the delay and has recently attempted to take his own life.

Fourteen months have elapsed since the boy was first identified by the Home Office as especially vulnerable and eligible for immediate transfer.

It has also emerged that Hammersmith and Fulham council in west London told the Home Office a year ago that it had a place for the teenager, but officials did not act on the offer – a decision that charities say has caused “irreversible damage” to the child, who has lost contact with his family in Syria. Giannoula Kefala, the council’s principal social worker, said: “From my perspective, the impasse and likely irreversible harm already caused to this extremely vulnerable child is unbearably disturbing.”

Kefala said that last December she informed the Home Office of her intention to travel to Greece to assess the boy. “It is absolutely clear from my visit that the long delay has caused this child terrible harm, and that it has been apparent for a long time that the available resources in Greece cannot cater for this child’s needs. Recent hospital records make clear that the ongoing uncertainty is having a devastating impact.”

The teenager is currently on heavy psychiatric medication, which worries his doctor but which is believed to be necessary to prevent a fatal outcome.

Until last Monday the youngster was being detained in a police cell with no access to medical professionals, and forced to sleep on a mattress on the floor. On 22 October, police said the boy, after repeated self-harming, had made a suicide attempt and was at “imminent risk of killing himself”. Kefala said she was concerned the boy could die.

The child’s imminent transfer is expected to precipitate a handful of similarly urgent cases. Almost 300 unaccompanied minors in Greece were identified last year as being eligible to be moved to the UK under the Dubs amendment to the Immigration Act, which was passed in April 2016 following a campaign to bring 3,000 lone refugee children stuck in camps to Britain.

Yet controversial changes introduced by the Home Office in March reduced the number of those eligible to come to the UK to about 40, excluding many who had previously been told they would be given sanctuary.

As a result, only four eligible child refugees have been identified in Greece since April, despite data showing 9,700 unaccompanied children had entered this year, most having fled Syria.

As frustration has mounted over the total failure to transfer child refugees from Greece, charities say they have repeatedly asked the Home Office for clarity over how the Dubs amendment is being implemented, along with its selection procedures and timeframe.

Antonia Moustaka, a lawyer for the humanitarian agency Praksis, said: “Many children on the Dubs list have run away, having lost hope after long-promised transfers never materialised.”

The youngster who is due to reach London this week was identified as a Dubs candidate in August 2016 because of his vulnerability, although Praksis only very recently learned that he was finally being transferred.

During his detention in Greece, the child has spent more than 380 days in psychiatric clinics, 124 days in shelters for unaccompanied minors, and six weeks in police detention.

One reason why his transfer is imperative is that there is no specialised facility in Greece to care for refugee children facing severe mental health issues. Conditions for unaccompanied minors have been condemned by Human Rights Watch, which found filthy cells infested with bugs and vermin, sometimes without mattresses or access to showers.

Charities have accused the Home Office of being more preoccupied with public relations than facilitating the boy’s transfer, questioning the effectiveness of an official deployed to Greece last year with the aim of facilitating Dubs.

Natasha Tsangarides, senior field manager for the charity Safe Passage, which is working alongside the UK government to relocate child refugees, said: “It is devastating that this child has reached absolute breaking point, facing a real risk of death, and has still not been transferred.”

Despite the youngster’s current condition, Kefala remained optimistic that the child, once in the UK, could improve. “He’s a very intelligent child in desperate need of affection, and he has deeply moved everyone who has met him. I share their optimism that, with suitable care and therapeutic arrangements, the boy’s impressive skills and coping mechanisms can be tapped to provide him the chance to lead a healthier and dignified life that every child deserves.”

Aside from the situation in Greece, 11,186 unaccompanied minors are known to be in France along with 13,867 in Italy, most of whom arrived by sea from north Africa.

A Home Office spokesperson said: “We remain committed to transferring 480 children from Europe to the UK under section 67 of the Immigration Act. We have accepted further referrals this year and transfers are ongoing. We will continue to work closely with EU partners and local authorities to transfer eligible children here quickly and safely.

“We are working closely with the Greek authorities and the UNHCR to put in place processes for identifying and transferring unaccompanied children from Greece under section 67 of the Immigration Act 2016 and the Dublin Regulation.”