Data compiled by Unison from 180 local authorities that provide youth services in the UK found that in 2014/15 and 2015/16 cuts amounted to £123m.
The union said that combined with previous Unison research, which found spending cuts of £264m between 2010/11 and 2013/14, it estimates cuts in youth service spending to stand at £387m since April 2010.
Unison's A Future at Risk: Cuts in Youth Services report said the financial year for 2014/15, which saw £85m cut, was a particularly bad year for councils, youth service spending.
"Two authorities, Lancashire and Liverpool, were forced to cut more than £2m each, while Essex, North Lanarkshire, Devon and Staffordshire all reduced spending by more than £1m," the report says.
"The following year, 2015/16, saw a further £38m in cuts, with Kent, Leeds and Staffordshire all removing more than £1m from their youth service budgets."
The report also found that around 3,652 youth work jobs were lost and 603 youth centres closed between 2012 and 2016. Meanwhile 97,909 places for young people were cut between 2014 and 2016 on top of 40,989 places between 2012 and 2014, making a total of 138,898 lost places.
The report goes on to warn that the future of youth services is "bleak" unless the cuts in central government funding can be "arrested and reversed".
"In the year 2016/17 and beyond, there is likely to be at least £26m more cuts in youth service spending, the loss of around 800 more jobs, more than 30 youth centres closed, and 45,000 more youth service places for young people removed," the report adds.
"This will only be the tip of the iceberg. Central government funding for local authorities has been slashed since 2010, and by 2021, with the abolition of the revenue support grant, authorities will be left to sink or swim on their own, relying on business rates and council tax revenue.
"Clearly, councils have difficult decisions to make, but while urging the government to change course and fund councils properly, we would also want to impress on local authorities the value and benefits of investing in these vital public services."
The union has called for provision of youth services to be made a statutory duty and for fairer funding to be introduced. It also wants government of all levels to involve and consult young people in decision making, services to be saved and kept in-house, and for public service workers to receive fair pay.
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 antisocial behaviour. It's vital these public services are protected."