Hundreds of migrants who were classed as adults after claiming to be children could have their cases reviewed after the Home Office's age assessments were ruled unlawful.
On Wednesday, the High Court said the way the Home Office assessed the age of migrants, using social workers to carry out quick judgments without an adult present and while they were under detention, was illegal.
Migrants who have been dispersed to adult asylum centres since the tests were introduced in September 2020 could now challenge the decisions.
The tests are necessary as adult migrants are known to claim to be unaccompanied children because they are more likely to be granted asylum and less likely to be detained or removed if their claim is rejected.
Channel people-smugglers are suspected of exploiting legal loopholes to encourage young-looking migrants to destroy their documents and claim to be minors amid a surge in crossings to a record high of more than 28,300 last year.
The new age assessments are understood to have been introduced in September 2020 as the number of people crossing the channel increased. They were effectively shortened versions of the established Merton tests, which involve two trained social workers deciding whether migrants claiming to be children are legitimately under 18.
However, the High Court ruled that the new guidance was "not lawful" after lawyers representing migrants said the "truncated process operated without observing basic fairness or providing young people with an appropriate adult".
Around 3,000 migrant children arrive in the UK each year, with up to 1,000 thought to have been affected by the assessments.
One who arrived in Britain, separated from his mother and having being locked in a lorry without much air and little food, was detained on arrival before being assessed for just 42 minutes.
Social workers decided, without an adult present, that he was 20, without providing him with any reasons or documentation. He was transferred to Yarl's Wood immigration removal centre before being released into adult asylum accommodation.
Another who arrived soaking wet, tired and terrified after an eight-hour journey across the Channel in a dinghy was assessed for an hour and determined to be 21. There was no adult present during the assessment, after which he was transferred to an immigration removal centre for five days without any explanation for the decision.
Enver Solomon, the chief executive of the Refugee Council, said: "We are relieved that the practice of hasty decisions is no longer allowed to continue. Distinguishing between adults and children is not something that can be done quickly – it takes time and expertise to make the right decision."
The Government is revamping the testing regime, reducing the age appearance threshold for when an assessment can be triggered from over 25 to over 18 and introducing more scientific methods for determining age.
A Home Office spokesman said: ‘’We are disappointed by the court’s decision. The Government is committed to protecting children and the vulnerable but we cannot allow asylum seeking adults claim to be children – this presents a serious safeguarding risk.
“Our Nationality and Borders Bill seeks to improve the challenging age assessment process and will widen the evidence base for social workers to consider when making assessments and lead to better informed decisions.”