The council says it aims to protect services as it makes savings
A council has sent letters to staff "reminding" them of its voluntary redundancy scheme as its says it will have to cut £21m from its budget.
Cardiff council blames an expected "poor settlement" from the assembly government and the recession.
The authority says it is still early in its budget-settimg process and it aims to protect "frontline services".
The Welsh Local Government Association (WLGA) has warned Welsh councils face making cuts of tens of millions.
Cardiff County Council said it had identified savings of £51m over the past five years and the average rise of 3.1% in council tax over this period was the second lowest in Wales.
However, the council says it is facing growing pressure in a number of key areas including the increasing of costs of adults living longer and demanding more social care, pension costs and the impact on revenue and benefit payments as a result of the recession.
In addition to the expected budget settlement from assembly ministers, it said it expected lower income from venues and the loss of interest payments as a result of the credit crunch.
The council's executive says it will need to find £21m in efficiencies in the 2010/11 budget.
Councillor Mark Stephens said: "The public sector is not immune to the effect of the global downturn which is why we are putting in place measures to tackle what we know is going to be a very tough budget round for the next financial year at this early stage.
"We know the challenge is even greater given the additional savings announced by the chancellor in April and the weakness of UK Government public finances.
"In the past we have talked of the need to make tough decisions and we will continue to do so.
"But at some stage, perhaps sooner than many would appreciate, the decisions become impossible without a genuine debate on the future role of the public sector within our society."
This month the WLGA said councils across Wales faced making cuts to cope with reductions in government funding.
The Welsh Assembly Government responded by saying it wanted "all public sector bodies to deliver efficiency savings which will safeguard frontline services".