Thousands of Jobcentre and benefit office workers are expected to join a 48-hour strike over job cuts.
Ministers remain committed to the planned changes
The Department for Work and Pensions (DWP) plans to axe 30,000 posts by 2008 as part of moves to modernise services.
But the Public and Commercial Services union (PCS) said services were already being affected across the UK by the loss of about 15,000 jobs since 2004.
The DWP said robust contingency plans were in place to maintain services while industrial action took place.
Its statement also said ministers remained committed to the programme of change.
The walkout on 26 and 27 January comes after talks broke down between the DWP and the unions over the cuts.
The strike is expected to disrupt job centres, benefit offices, pension centres and the Child Support Agency.
The union claims services have been on the verge of collapse, with delays and computer problems, following the loss of 15,000 jobs.
It says telephone calls to Jobcentres are going unanswered, while people are having to wait hours to see an adviser.
And staff are facing an increasing number of assaults because of frustration at delays, the union claims.
It had called on the programme of cuts to be halted to allow a proper evaluation of staffing levels.
It wants a guarantee of no compulsory redundancies.
PCS General Secretary Mark Serwotka accused managers of burying their heads in the sand and pretending everything was fine.
Chancellor Gordon Brown announced the redundancies in March 2004, as part of wider civil service cuts aimed at boosting funding to frontline services such as doctors and nurses.