Savage cuts will lead to more people serving short jail terms, the probation union has said.
Harry Fletcher, Napo's assistant general secretary, said 25% cuts to the probation service budget will lead to a lack of resources and courts being forced to hand down short prison sentences despite the Government's plans for a rehabilitation revolution.
He added that it was "inevitable" that probation officers will vote for strike action at their annual conference in Scarborough, and they could join with other unions in "days of concerted action".
Justice Secretary Kenneth Clarke wants more offenders to be given community sentences rather than short jail terms, but Mr Fletcher said this could be compromised if Government spending cuts of 25% are announced on October 20.
Napo said it believed courts would be forced to use short prison sentences if fewer resources were available for community orders.
Mr Fletcher said: "The immediate effect will be redundancies, fewer court reports recommending non-custodial options, and a rise in the short-term prison population.
"Napo members are committed to the concepts of rehabilitation and protecting the public but these aims will be compromised when the cuts take place."
He added: "Industrial action is inevitable and will involve joint initiatives with other public sector unions. It is ironic that if the coalition invested in the Probation Service in the short term, it would save massive amounts in the medium and long term. The Government should invest in probation and not cut and privatise it."
Napo said it expected the number of probation staff to fall from 20,000 to 15,000 by March 2012.
A Ministry of Justice spokesman said: "Public protection is one of our main priorities and there is no evidence to suggest that savings made to the Probation Service will compromise this or increase crime."