Cabinet Office minister Francis Maude has called the PCS' decision to strike "futile, counter-productive and irresponsible."
He added that the decision to strike benefits no one, but "threatens the services people rely on."
He said: "I can reassure the public that the Government has done everything we can to minimise disruption [today]. Rigorous contingency planning is in place to try and limit the impact of the strike action and to ensure that affected public services operate as effectively as possible."
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An ejector seat manufacturer is to be prosecuted over the death of Red Arrows pilot Sean Cunningham at RAF Scampton in November 2011.
Police said the "active shooter", reportedly a lawyer, had since been shot by officers and they were not seeking any other suspects.