The Home Office is routinely changing the dates of birth of unaccompanied child asylum seekers to classify them as adults, according to experts who say the practice is now happening on a “horrifying scale”.

As a result, many children are being wrongly sent to the notorious Home Office site at Manston in Kent, the experts warn, and detained in unsafe conditions for up to several weeks.

The Refugee Council said interviews with 16 children released from Manston revealed that even in the cases of some boys who had identity documents stating they were children, the Home Office changed their dates of birth to make them over 18.

One of the children, interviewed at a hotel in London last week, said they had been attacked by adults in Manston, and others described fights and the police being called. 

Sonia Lambert, who works for the Refugee Council, described boys giving their ages as 15 to 17, but Home Office staff not accepting their stated ages. One had his birth date altered by one year, putting him over the dividing line of 18. “I cried so much but they still didn’t change it back,” said an Afghan boy.

Three of them showed images of ID documents or passports on their phones to officials from the Refugee Council’s age-dispute project, which had been either ignored or discounted by the Home Office. Some of the boys said they could not understand why they had been “given” a new age despite their protests.

Categorising the unaccompanied child asylum seekers as adults meant some were placed in Manston for more than 10 days at a time, and several for over 20 days.

Manston was emptied last week following a series of controversies at the site, including overcrowding, allegations of drug selling by guards, asylum seekers being left stranded in central London after their release from the centre, and outbreaks of infectious diseases, such as diphtheria.

On Saturday the Home Office said that the recent death of a migrant held at Manston may have been caused by diphtheria, a highly contagious infection that affects the skin, nose and throat.

Lambert, who was among those who visited the hotel where the children were interviewed, said: “One of the most shocking things to me is how young many of them looked. Several even had proof of their ages, and showed us images of ID cards or passports, which seem to have been ignored or discounted by Home Office staff.”

Of the 16 interviewed, eight said they were 16, six said they were 17 and two 15. Three had documents which appeared to prove their age, but had been ignored.

One of the 15-year-olds, from Iran, had an image of his passport and his ID card, but said that Home Office officials were “not interested”. He claimed they told him it didn’t matter that they’d given him a new date of birth because there would be a chance to correct it later on. The boy went on to spend 24 days in Manston.

One of the boys, from Syria, said: “I’ve never been more frightened than I was in Manston.” Another, from Sudan, added: “It’s the worst place I’ve ever been to.”

Renae Mann, executive director of services at the Refugee Council, said: “This is a misuse of power by the government. These children are very vulnerable and have been through so much already.

“Our staff are seeing many young people aged 15, 16 and 17 who have been wrongly treated as adults by Home Office staff. It is only once we get involved that these children are recognised as such and are taken into local authority care.” Once classified as adults the boys are at risk of being “dispersed” to adult accommodation, usually hotels, throughout the UK.

The Refugee Council has worked with 92 young people at the same hotel since the start of September, all of whom it has successfully managed to have taken into care. Mann added that the government had not published data that offered a clear picture of the numbers of children affected.

“As the government works to address the huge challenges in the asylum system, the treatment and welfare of children must be prioritised.”

A Home Office spokesperson said: “Age assessments are challenging but vital to identifying genuine asylum seeking children and stop abuse of the system. We are taking a holistic approach to prevent adults claiming to be children, or children being wrongly treated as adults – both present serious safeguarding risks to children.

“Our reforms through the nationality and borders act aim to make assessments more consistent and robust by using scientific measures, and creating a new national age assessment board. If there is doubt whether a claimant is an adult or child, they will be referred for a local authority assessment and will be treated as a child until a decision on their age is made.” By Mark Townsend, Guardian