Funding is at a "critical point" at a Welsh council which has warned services face being slashed.
Libraries, leisure centres, child and elderly care provision and school budgets could all be hit, leaders of Bridgend council said.
They have told a meeting they will soon have to take decisions on cuts that were previously "unthinkable".
The Welsh Government said it had taken action to protect local authorities.
Bridgend council's cabinet said the authority needs to find £10m in savings by the end of 2020, and a total of £30m by the end of 2023.
Council leader Huw David urged all councillors - regardless of political party - to write to their assembly members over the issue.
"In each and every council chamber in Wales, councillors are asking whether vital services can continue to be delivered in the face of continuing cuts and the answer is the same; further cuts are not sustainable," he said.
"Additional funds need to be invested in social services, schools, housing, parks, arts and culture, all the areas which keep our communities together."
The cabinet was told that if financial pressures were not relieved, the council would be forced to consider cutting "precious services" including:
- Closing libraries and community centres
- Closing leisure centres and swimming pools
- Ending full-time nursery education for three-year-olds
- Closing daycare services for older people and adults with learning disabilities
- Reducing budgets of primary, secondary and special schools
- Removing school crossing patrols
- Removing all local bus subsidies
The cabinet said the next budget settlement from the Welsh Government in the autumn needs to be inline with inflation.
Welsh Government officials said ministers have been working against "a backdrop of austerity as a result of years of successive and severe cuts" from the UK.
"Despite this, we have taken action to protect local authorities in Wales and have also ensured that no local authority will have to manage on a reduction of more than 0.5% this year.