Thousands of workers in magistrates and crown courts, tribunals, the Ministry of Defence and Forestry Commission are going on strike in a long-running dispute over jobs, pay and working conditions.
The Public and Commercial Services (PCS) union said its members across several Government departments and agencies will walk out for half a day in the latest phase of industrial action which has been affecting the civil service for months.
The union has launched a three-month campaign in protest at cuts to jobs, pay and pensions as well as changes to working conditions.
The PCS said lawyers campaigning against legal aid cuts will join hundreds of striking courts staff at a protest in Manchester from lunchtime.
Cleaners and security staff in courts will also take strike action over pay.
The MoD strike will include civilian staff who work alongside military colleagues at sites including the navy bases at Faslane and Portsmouth, RAF Cosford and Abbey Wood in Bristol - one of the largest civil service sites in the UK.
PCS general secretary Mark Serwotka said: "These workers provide essential services and support across our justice system and in defence, and they deserve better than being treated with contempt by a Tory-led government that is refusing to even talk to us about the cuts it is imposing.
"The joint protest in Manchester is a great example of different groups coming together in solidarity to fight for a shared cause and we would like to see more of this happening across the country."
A Cabinet Office spokesman said: "We are grateful to the overwhelming majority of public sector workers who will remain at work today to deliver essential services.
"It is disappointing that yet again the PCS insist on pushing for futile action which benefits no-one, and damages the services they deliver to the public."