UK government data has revealed that 52 migrants who were later identified as being over 30 managed to dodge initial checks and claim to be children.
Between 2012 and 2020, an average of 355 migrants a year claimed to be children who were later found to be adults.
The child age data plays into concerns that people traffickers encourage asylum seekers to pretend to be children, saying they will be treated more generously.
Even though the figures show that only a tiny minority of migrants claim to be children, there are fears that many more escaped detection.
More than 20,000 people have been recorded crossing the English Channel in small boats so far this year. Another 606 migrants reached the UK on Wednesday, according to the most recent data available.
There were 1,500 cases of adults of any age pretending to be children in 2021 and 364 in 2020.
Migration Watch, a group which supports lower migration into the UK, said the statistics mean vulnerable children are being exposed to further dangers.
“These are astonishing figures that reveal a seriously worrying situation. There is absolutely no excuse for placing people much nearer middle age than childhood among vulnerable minors,” said group chairman Alp Mehmet.
“No individual, where doubts about age exist, should be allowed anywhere near vulnerable youngsters, in any setting. The safety of our children has to be paramount.”
The number of people reaching the UK in small boats from France, navigating busy shipping lanes, has increased sharply in recent years.
A total of 299 were detected in 2018, followed by 1,843 in 2019, 8,466 in 2020 and 28,526 last year, official figures show.
Despite the growing numbers, the UK’s small boat arrivals are a fraction of the number of people migrating to Europe.
Data from the UN’s refugee agency shows at least 120,441 people arrived in Europe via the Mediterranean by land and sea in 2021.