A report by Coram Children's Legal Centre (CCLC) highlights the vulnerability of children who live in the UK but who do not have citizenship or indefinite leave to remain. A study in 2012 estimated there was 120,000 "undocumented" children in the country, 65,000 of whom were born here.
In addition, CCLC is concerned that Brexit poses the potential to significantly increase the number of undocumented children and young people.
The government, which embarked on Brexit negotiations last week, has pledged citizenship rights to European Union (EU) nationals, including children, who have lived in the UK for a minimum of five years. However, CCLC is concerned that if this fails to materialise, thousands of children of EU nationals could become undocumented once the UK leaves the union in 2019.
Lack of legal immigration status means these young people are unable to attend college or university, work, receive housing support, claim benefits, open a bank account or hold a driving licence.
The CCLC report, This is My Home, says being undocumented puts children and young people at greater risk of poverty and exploitation.
It says the current immigration system has a number of legal and cost barriers in place that prevent parents of undocumented children applying for legal status.
Kamena Dorling, author of the report and head of policy and programmes at Coram, says the current system is broken and needs changing to make it easier for undocumented children to gain legal status.
"Thousands of undocumented children and young people will have grown up in the UK, have been educated here," she says. "Even if they manage to make an immigration application, they will usually only be granted two and a half years leave and will have to wait 10 years before they can apply for indefinite leave to remain."
Dorling adds that a "simple, straightforward process" enabling EU national children to obtain settled status post-Brexit is needed, otherwise "the number of undocumented children will triple".
The report recommends an urgent review of children and young people's need for legal support, changes to Home Office policy and decision making and better information for social workers.
Enver Solomon, director of external affairs at the National Children's Bureau, said: "The Brexit negotiations will affect the lives of children and young people living in the UK for many years to come. For some children born to EU nationals, Brexit creates acute uncertainly about whether and for how long they can remain in the country they call home.
"The government should make clear that decisions about where a child lives will be guided by their best interests - not simply their parents' employment history. The deal must also be fair to vulnerable children and those from single parent families."