Careers services for young people in England are being slashed by up to 50% which could lead to 8,000 job losses, the public services union Unison says.
Cuts in grants to local authorities for youth services could mean one in four staff at Connexions careers advice services lose their jobs, it says.
The cuts come as young people face record competition for university places and a difficult jobs market.
Prime Minister David Cameron said Connexions had not been very effective.
Unison said cuts to the service ranged from about 10% to 50% in different local authorities.
"It's devastation," said Steve Higginbotham, vice president of the Institute of Career Guidance.
As the cut, about 24% of the "area based" grant which funds Connexions, is being enacted halfway through the financial year, it means in practice up to 50% losses to budgets for the remainder of the year, he said.
Connexions offers advice on careers, education, work and personal issues to people aged 13 to 19.
It has been criticised for focusing too much on young people who are not in employment, education or training - referred to as "Neets", at the expense of the needs of more mainstream teenagers.
Mr Higginbotham said some schools could come back after the summer break to find Connexions services unable to fulfil their commitments.
Meanwhile, many young people who did not get the grades to secure university places would face "very difficult decisions", he said.
On Tuesday, the head of the university admissions body Ucas said thousands of school-leavers would be turned away from university this year, and should consider apprenticeships instead.
Unison official Denise Bertuchi said: "We are facing a time when there will be an increase of young people who will not make the grades required for FE (Further Education) or university because the bar has been raised in the GCSE and A-levels."
"We currently have 2.5 million looking for work and five jobseekers for every vacancy," she said.
"This is a major failing of this government not only in the reforms being proposed for education but in the flaws in the decisions to reduce the area-based grant without due regard for the consequences."
According to Unison, some of the hardest hit areas are:
- Norwich, where 50% cuts will lead to 65 job losses
- Connexions Northumberland, which says 24% cuts will make redundancies "inevitable"
- Council-funded youth advice charity Sheffield Futures, which says up to 95 of 370 jobs could be lost
- Sutton - 34% mid-year cut, said to equate to 50-60% of current activity
- Cheshire East - losing a third of front-line workforce
- Bolton, where the cuts equate to about 30% of activity
In Norfolk, where there have been protests against the cuts, the council's cabinet member for children's services, Alison Thomas, told local media that the cuts were "unavoidable" but there was scope to look at delivering "an effective service at a reduced cost".
She said guidance would be focused on vulnerable young people, with wider advice "largely web-based".
Speaking to BBC West Midlands, Prime Minister David Cameron said the Connexions service had "not been a great success story".
"Many analyses have been done of it in terms of it not being very good value for money," he said.
He said the government would be introducing "a proper work programme" which would "make sure that young people get the help and the training that they need to get into a job."
A Department for Education spokesman said: "We have been clear that it is for local authorities to manage reductions to area-based grants across all their funding to protect front-line services. We have removed the ring-fencing of funding where possible to give local authorities maximum flexibility to find efficiency savings in ways that respond to local needs."
It said it would set out plans for improving the provision of careers advice in the autumn.
A selection of your comments are below
Connexions is absolutely useless - had a careers "interview" in Year 10 through school. Did not help at all. Did a silly quiz as well which said I should aim to become an opera singer - interesting as a cat has more vocal talent than me.
Anon, Hexham, Northumberland
I have two young teenagers who have benefited greatly from various Connexions services. Any cuts will, in my opinion, lead to more teenagers not having activities to take part in and this will lead to teenagers getting into trouble and not having any job focus. I find the Connexions centres to contain knowledgeable staff who give young people quality support and guidance.
Thomas Russell, Carlisle
Connexions as a service has changed vastly. Gone are the days of equality for all young people as government agendas have put the nail in that coffin ensuring that the NEET generation is the main focus. Advisers have been constantly pulled away from institutions where young people need guidance to reduce the amount of young people who are NEET.
Mark Lee, Co. Durham
I can't believe David Cameron said that Connexions has not been very effective! Who does he think has been keeping figures low for young people who are NEET.
Emma Hogget, Cheltenham Gloucestershire
Good. At last a service in need of cuts. I am now a year 13 college student and the only time Connexions seemed to bother with me after and during my GCSEs was a week before I started college. They are a waste of money and time. I have spoken to them once since my GCSEs and what rubbish they told me was useless. Even my teachers completely disagreed with them.
Matthew, Rugeley, Staffordshire
After leaving school my son went to them for advice. He wanted to go to university but hadn't quite enough points to get in for his chosen course. He had to make an appointment with them, but as he couldn't make the set one, he contacted them to make another but never did get one! We gave up trying to organise an appointment and he had a year off with no study, work or benefits and we worked out how to get him into college ourselves. He starts in September.
Mags, Reading, UK
I have worked for Connexions for eight years. We have had bad press but it was the government of the time that dictated we target the NEET group. We are losing very skilled people right across children's services and many of those services are to very vulnerable young people. It concerns me that this government says they are going to look at ways to improve the service in the autumn but the staff will have been made redundant by then.
Gill Cassidy, Hampshire
The problem with Connexions is that it was actually underfunded from the very beginning. The size of the challenge - to engage, listen to, guide, inform, advise and enthuse all young people - whatever their level of skills and whatever their level of exclusion, disaffection and/or disability - in a working world requiring ever higher levels of technical and people skills was completely underestimated and has been met on the cheap. But that should not be an excuse to close us down. I think we've actually done rather well, considering.
Terry Miles, London