Thousands of tax workers will stage strike action in separate disputes over jobs and privatisation of services and a "punitive" new sickness absence system.
Members of the Public and Commercial Services union (PCS) at HM Revenue and Customs will walk out for three hours from 12 noon in protest at plans to hand "sensitive data" to private companies, which they fear will lead to job cuts.
The union said around 20,000 of its members will be involved in the industrial action as it steps up opposition to a planned year-long trial using private staff in two contact centres in Bathgate, Scotland and Lillyhall, Cumbria.
PCS officials said the private staff will handle enquiries about tax credits and will have access to sensitive data about people entitled to claim them.
The union said that instead of cutting more jobs at a time of high unemployment, and "wasting" public money on a private sector trial, the department should use existing staff to help out at peak times in the call centres.
In a separate dispute, all the union's members in HMRC's offices across the UK will walk out between 12pm and 2pm on the same day in opposition to a new sickness absence system, which the PCS said threatens staff with disciplinary action instead of supporting them back to work.
The latest action over the new "attendance management" policy follows a series of short walkouts over two days in June, which closed some tax inquiry centres.
PCS general secretary Mark Serwotka said: "Our members have very serious concerns about this creeping privatisation into HM Revenue and Customs and, as well as the threat to their jobs, they do not think such sensitive data should be handed over to private firms.
"This is also happening at a time when senior managers are trying to bring in an unnecessarily punitive and counter-productive sick policy that seeks to penalise people for being ill, rather than support them."
The union has more than 50,000 members at HM Revenue and Customs.