New research estimates that more than 100,000 children are living in London without secure immigration status, despite more than half of them having been born in the UK.

Children who are undocumented may face problems accessing higher education, health care, opening bank accounts, and applying for driving licences, housing and jobs. The findings were condemned by the mayor of London, Sadiq Khan, as a “national disgrace”.

The study, commissioned by the mayor and undertaken by the University of Wolverhampton, estimated that there were around 107,000 undocumented children and 26,000 18- to 24-year-olds in London. Once an undocumented child turns 18, they face the threat of deportation to a country they may never have visited.

Undocumented people can include those who arrived in the UK with proper documentation but who stayed beyond their permitted time, those who entered without proper documentation, trafficked children, unaccompanied minors whose temporary leave to remain was withdrawn once they reached adulthood and young people born to parents who are themselves undocumented.

The research found that more than half of the UK’s estimated 674,000 undocumented adults and children live in London. It warns that the number of undocumented young people could rise dramatically if the estimated 350,000 young European nationals in the UK are not helped to apply for the EU Settlement Scheme that will enable them to remain after Brexit.

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Assessing the size of Britain’s undocumented population is inevitably a challenging process, since there is no official data and it requires counting people outside most formal systems. Instead, the report has reviewed previous research and analysed all available data to come up with a conservative estimate. The study suggests that the population of undocumented migrant children has grown by 56% between March 2011 and March 2017.

The report highlights the high cost of regularising immigration status. “The Windrush scandal has exposed the barriers facing people who have lived in the UK for many years, including a complex application process, a lack of awareness of the system, cuts to legal aid and the high cost of applications – with the high court last month deeming as ‘unlawful’ a government decision to charge £1,012 to register children as British citizens,” the report states. “Since 2012, only 10% of families with undocumented children in the UK have applied to secure their immigration status.”

This finding echoes research by the Let us Learn campaign group published last year, which found that the cost of renewing an application for a resident with limited leave to remain has increased by 238% in five years – from £601 per person in 2014 to £2,033 in January 2019. The organisation had seen cases where parents were forced to choose which child’s status they could afford to maintain, without enough money to pay fees for the whole family.

Khan said: “It is a national disgrace that there are hundreds of thousands of young Londoners being denied the opportunity of a secure future in our city and living in constant fear of deportation from the government’s hostile immigration policies. These young people, many of whom were born in the UK, are often unable to access higher education or work, to rent a home or open a bank account, and these numbers are set to grow dramatically when Britain leaves the EU.”

Kamena Dorling, head of policy for Coram children’s charity, said: “UK citizenship and immigration policy is failing a significant number of children who have grown up in the UK. These children are growing up in limbo instead of being legal citizens in the country they call home. No citizenship and immigration system can succeed if it excludes this many of the country’s children and teenagers from legal status.”

A Home Office spokesperson said: “We do not recognise the figures quoted in the report. There are a range of routes and options available for people of all ages to regularise their status, including, children who have lived in the UK for most of their lives.

“We do not agree with the notion that leaving the EU will increase the number of undocumented children. The EU Settlement Scheme allows applicants, including children, to apply without an identity document when there is a reason beyond their control why they can’t obtain one.”