Cuts will lead to more people serving short jail terms, the probation officers' union has claimed.
Napo said if there were 25% probation service budget cuts this would lead to courts giving short prison sentences.
Assistant general secretary Harry Fletcher said it was "inevitable" members would vote for strikes at their conference in Scarborough this week.
A Ministry of Justice spokesman said savings would not compromise public safety or increase crime.
Justice Secretary Kenneth Clarke wants more offenders to be given community sentences rather than short jail terms, but Mr Fletcher said this may not be possible if government spending cuts of 25% are announced on 20 October.
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 the probation service must make savings, like every other area of government.
He said: "Public protection is one of our main priorities and there is no evidence to suggest savings made to the probation service will compromise this or increase crime.
"Any savings will look to retain frontline services which will ensure the public is protected and re-offending is reduced.
"As part of our assessment of sentencing and rehabilitation policy, the coalition government is also looking at how private and voluntary sector providers can get involved in running rehabilitation services to make them tougher for criminals and better value for the taxpayer.
He added that although the Ministry of Justice had submitted proposals for savings to the Treasury, a final decision had not been made.