Plans to outsource the collection of court fines have been "snuck out" by the government, putting 150 civil service jobs at risk.
HM Courts & Tribunals Service said it was in talks with "providers", adding that the move would save millions of pounds.
But the Public and Commercial Services union (PCS) opposed the change, warning that any private company given the work would want to make savings, which could affect jobs.
General secretary Mark Serwotka said: "The last time ministers tried to do something similar it ended up costing taxpayers £8 million before being abandoned, now they're trying to avoid scrutiny by sneaking it out during the summer holiday.
"This work is highly sensitive and should remain in-house instead of being handed to private bailiffs whose motive is profit."
A Courts & Tribunals Service spokesman said: "We take the recovery and enforcement of court fines very seriously and it is vital that offenders either pay or are brought back before the court.
"While no decisions have been made, we are in discussion with providers to extend the work of approved enforcement agencies which would save the taxpayer more than £18 million over the next five years."
Richard Burgon MP, Labour’s Shadow Justice Secretary, said: "It is unbelievable that the Government is planning further privatisation to the justice system.
“The outsourcing of Civil Enforcement Officers will combine weaker oversight with pressure to maximise profits and risks more and more people becoming victim of rogue private bailiffs.
“A previous attempt to privatise all enforcement work was abandoned, but only after the Conservative government had wasted millions of pounds of taxpayers’ money pursuing it. It should not be putting such a flawed idea back on the table.
“From the deep failures in the part-privatised probation system to the scandal surrounding G4S tagging contracts, the Conservative Government's obsession with privatisation is at the expense of delivering a fair justice system that works for all."
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