Hundreds of millions of pounds have been cut from youth services across the UK in the past six years, leading to the loss of more than 3,600 jobs, according to a new report.
Unison said a study showed most local authorities had cut services such as youth centres, advice and outreach support.
This has led to the loss of around 140,000 places for young people and the closure of 600 youth centres since 2010, the research found.
The report, based on a Freedom of Information request to local authorities, said the outlook for youth services was "bleak".
Unison general secretary Dave Prentis said: "Youth workers are tireless in the support they provide to young people, for example helping them find work and getting more education.
"It's youth services which prevent problems happening in the first place by reducing feelings of isolation among young people and helping teenagers to lead positive lives.
"But they've been relentlessly cut and undermined at a time when they are needed more than ever. This is damaging young people's life chances, especially those from poorer backgrounds, and raising the risk of mental illness as well as anti-social behaviour. It's vital these public services are protected."