Economic inactivity among 18 to 24-year-olds due to ill-health has nearly doubled over the past decade, according to new research.
Most of those are struggling with education, with four in five young people who are too ill to work having only qualifications at GCSE-level or below, said the Resolution Foundation.
The think tank said the "worrying trend" has gone completely under the radar.
Its study, funded by the Health Foundation, said that overall levels of worklessness among young people are low.
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In early 2023, the number of young people not in education, employment or training (Neet) stood at 720,000, lower than the post-financial crisis peak of 1.1 million.
But there was a near-doubling of the number of young people not working due to ill-health, from 94,000 in 2012 to 185,000 in 2022, said the report.
Almost one in four workless young people are inactive because of ill-health, up from less than one in 10 in 2012, the study indicated.
The Resolution Foundation claimed that policymakers' attention has been focused on rising ill-health among older workers.
Spokeswoman Louise Murphy said: "Overall worklessness among young people is currently low, but beneath this welcome headline trend lies a worrying rise in the number of young people who are not working due to ill-health.
"Worklessness due to ill-health among young people is most common in small towns and villages, but reflects these young people's low levels of education far more than the nature of their area.
"This highlights the protective effect that education can have on a person's ability to access mental health support, and to succeed in the labour market.
"We cannot afford to let young people who are workless due to health problems get left behind, so we need both to improve their education opportunities and to ensure that everyone has access to better mental health support."