Up to 1,300 jobs are set go at a county council as it looks make savings of £79 million over the next three years.
An estimated 500 posts - 100 of which are temporary agency staff - will go at West Sussex County Council between now and 2014.
The Conservative-led authority also said a further 800 jobs may be transferred to employment with other service providers.
The cuts will see the total number of people employed at the council, excluding schools, fall from 6,300 now to 5,000 in three years.
County council leader Louise Goldsmith said making the cost-savings decisions "was the worst times for all of us".
The county council said most of the jobs to be lost will be in management and back office and support services.
Roles will also be shed in office administration and specialist functions such as human resources, training, finance and democratic services.
Other positions will go as a result of changes which have been subject to consultation, including in the youth service, as a result of the closure of the under-occupied Littlecote Children's Home and changes in the learning service.
A council spokesman said: "Staff and unions have already been told of the likely impact of the measures the county council needs to introduce over the next three years.
"In total, excluding schools, the county council will be employing some 5,000 staff in three years' time compared with 6,300 at present."
It is hoped that compulsory redundancies will be avoided by losing posts through retirement, resignations, redeployment and voluntary redundancy, the council added.
Ms Goldsmith said: "Unlike some other councils, we have not announced job losses first but have worked through all the implications of the proposed savings, linking them with our future objectives.
"It has been the worst of times for all of us.
"Nobody came on to the county council to make savings of such proportions, yet each one of us has to face our responsibility.
"These savings cannot be made without radical changes to the county council and staff.
"I know that we have a committed workforce who do their best for the organisation and for the residents we serve and I thank each and every one for all they do.
The most difficult task for me today is to announce the implications in terms of job losses.
"The council has been preparing for this day and over the last year or so has been holding posts vacant in order minimise the impact on our staff."
Unison West Sussex branch secretary Chris Earwaker said it was a worrying time for all affected employees.
She said: "From the point of view of redundancies, we are hopeful that a significant proportion will be generated by people who want to go voluntarily.
"But what that will leave behind could cause problems in terms of a significantly reduced workforce and stress and pressure on those left behind."
Ms Earwaker said the union planned to challenge the council over its plans to outsource employment.
"They are saying that they aim to save somewhere in the region of £5 million by outsourcing but we don't think that is right and we will be challenging them on their business case," she said.
She went on: "It's a very worrying time but it affects people from across the board at the council.
"It's not just confined to one area."
Ms Goldsmith said: "We absolutely recognise that these are very difficult times for our staff and their families, and we are working to handle the very necessary changes we have to make as fairly and sympathetically as we possibly can."