Campaigns, including strikes, against civil service job cuts are to be launched across the UK.
The Public and Commercial Services union said it would also consider legal action against the government over cuts to redundancy terms.
The government's plans would mean civil servants lose about a quarter of their redundancy pay and cut tens of thousands of jobs "on the cheap", said the union.
Since 2010, 100,000 posts have gone from the civil service, according to the PCS.
A special meeting of the union's national executive committee agreed to launch campaigns including localised strikes, as well as co-ordinated action with other unions, politicians and the public.
Alongside this, the union is actively considering mounting a legal challenge.
PCS general secretary Mark Serwotka said: "This is further proof that far from being the party of the workers, the Tories remain wedded to the same failed austerity policies, seeking to axe tens of thousands more jobs on the cheap and damaging the services we all rely on.
"With previous cuts to these terms in recent memory, it is reprehensible that staff are being treated with such contempt, and ministers and senior officials have acted in a disgracefully underhand way by preventing any genuine negotiations.
"We are determined to fight the obvious consequences of these cuts with campaigns for jobs and public services in our communities across the UK, and through parliament and the courts if necessary."