LONDON (AFP) - Children like Adam and Nastashia are already scarred from the trauma of a troubled upbringing and being placed in care, but Brexit has placed another new terrifying burden on them.

Even though both were born in Britain, and are currently living in care there, because their birth parents were EU nationals, Adam and Nastashia risk becoming undocumented and losing their residency rights as a result of Britain's withdrawal from the 27-nation bloc.

And that means they may even face the threat of removal once they turn 18.

"They will not have the right to access free medical care, work, claim benefits, rent a home, learn to drive and hold a bank account," Marianne Lagrue, policy manager at the Coram Children's Legal Centre charity, told AFP.

Since the end of the Brexit transition period on January 1, EU citizens wishing to settle or stay in Britain have had to clear extra hurdles.

Although new European arrivals face tougher immigration rules, those who were already in Britain as of December 31, 2020 can retain their rights by applying to the EU Settlement Scheme by June 30.

The British government insists its scheme is a "success", with some five million EU nationals granted temporary (pre-settled) or permanent (settled) status - far more than the three million originally estimated.

However, some have slipped through the scheme's net.

"It is straightforward if you are in employment, digitally savvy, have all your documents and file," said Azmina Siddique, of The Children's Society.

But children placed in care, and young adults previously in care, face a tougher predicament.

Some struggle to prove their identity, supply proof of residency or receive the necessary support, which falls to their legal guardians or the local authorities.

Coram cites the example of Adam, a four-year-old Romanian boy who was born in London and separated from his mother.

Adam cannot obtain a passport from the Romanian embassy as the identity of his father - whose consent is required - is unknown.

Meanwhile, social workers have found it difficult to prove his place of residence before he was placed in care.

Then there is the case of Nastashia (not her real name), 17, who split from her family.

Although she, too, was born in Britain, she has no passport and encountered many difficulties when applying for the Settlement Scheme.

"Many of them may not realise they're not British," said Ms Siddique, adding that the impact can be "hugely traumatic" for the children and "really hold them back in life".

Given that the British government does not issue ID cards and the nationality of children in care is not recorded in Britain, it is difficult to know how many are encountering these problems.

According to Britain's interior ministry, 3,660 vulnerable young people up to the age of 25 have been identified as eligible for residency status, of whom 67 percent had applied by the end of April.

But charities and campaign groups say the figure is significantly under-estimated and claim there may be up to 9,000 vulnerable EU children in Britain.

A Home Office spokesperson said the government was "working closely" with charities and local authorities and had provided them with £22 million (S$40 million) in funding.

Late applications for the Settlement Scheme would be accepted "on reasonable grounds", they said.

Ms Siddique, however, believes this support does not go far enough.

From July 1, children who missed the Settlement Scheme deadline will be left with "no protection" until an application has been submitted and accepted.

The wait could drag on for years and expose the children to the government's "hostile environment" policy towards undocumented migrants, Ms Siddique said.

"These children could become the next generation of Windrush children," she warned, referring to a scandal in which Caribbean immigrants, who moved to Britain legally between 1948 and 1971, were denied rights because they were unable to provide the necessary documents.

Campaign group the3million, which represents EU citizens living in Britain, is demanding physical proof of settled status, but the government argues it is not necessary.

According to UK in a Changing Europe, a think-tank, hundreds of thousands of people could find themselves undocumented, including the elderly, the homeless, victims of domestic abuse and children whose parents mistakenly think their own residency rights cover their offspring.

"If the government cannot regularise the children in its own care, what about children in vulnerable families and vulnerable adults?" said Ms Lagrue.

"What hope do they have?"