Tens of thousands of workers from the two largest government departments will hold joint strikes this week in a long-running dispute over pay, pensions and jobs.
Members of the Public and Commercial Services union employed by the Department for Work and Pensions at jobcentres and benefit offices will join tax workers from HM Revenue and Customs in a series of regional walkouts from Monday to Friday.
The strikes form part of the union's three-month civil service-wide campaign of industrial action against spending and other cuts.
The union said the two departments are at the heart of the political debate about public spending.
PCS general secretary Mark Serwotka said: "These workers are the backbone of our country's public services and do not deserve to be treated with contempt by ministers who are refusing to even talk to us about the cuts they are imposing.
"Collecting even a fraction of the money we lose through tax dodging by wealthy individuals and organisations would make it impossible for the Government to claim it has no choice but to cut billions from the welfare budget for sick, disabled and unemployed people."
The union is also campaigning to stop Government plans to close all 281 of HMRC's walk-in tax advice centres in the UK and divert inquiries to jobcentres. Pilot closures of 13 inquiry centres in the north east of England starts on Monday.
The union said around 135,000 civil servants will be involved in this week's action, which is set to be followed by a national walkout at the end of the month.