The use of electronic tags to monitor curfew orders should be scrapped as they are too expensive and do not work, the probation officers' union has said.
More than 25,000 offenders were given tags in the past 12 months
Napo claims replacing the tags with supervision by probation officers would have saved about £110m over four years.
But the Home Office defended the firms paid to administer the scheme, saying they would get less money if their performance was unsatisfactory.
More than 25,000 offenders were tagged in the 12 months to April.
Napo assistant general secretary Harry Fletcher said: "Electronic monitoring is now a multi-million pound business set for a major expansion after the election, yet the figures clearly show that the profit is huge and hardly value for money.
"There is an overwhelming case for the withdrawal of the curfew order.
"The time is surely right for an independent assessment of this concept."
More than 25,000 convicted offenders were tagged and placed under the scheme in the 12 months to April.
Mr Fletcher said research by Napo indicated the government was paying private companies £1,700 for each tagging order, but the union believed the companies' outlay was no more than £600 per order.
The total amount spent by the Home Office on curfew orders and the home detention curfew scheme, which allows prisoners to be freed early, was more than £220 million over the last four years, Mr Fletcher said.
But he claimed the cost of supervising individuals by the Probation Service on community orders or parole was less than half the cost and delivered better results.
He added: "The taxpayer could have saved, therefore, over £110 million over the last four years if community alternatives had been used."
Probation staff have also reported that multiple violations of tagging orders were not being brought to court.
In the worst case, one offender violated the order 34 times before being returned to court, Mr Fletcher said.
A Home Office spokeswoman said: "Home detention curfew provides prisoners with a smoother and more effective reintegration back into the community.
"It enables them to be released from prison early while still subject to restrictions placed on their liberty.
"The performance of the private companies operating electronic monitoring across England and Wales is monitored closely by the Home Office and failure to meet agreed levels of performance will result in deductions from the amount paid for delivering the service."
Offenders given curfew orders have to stay at home for up to 12 hours a day, for a maximum of six months, and their movements are monitored via an electronic tag fitted to their ankle.
The Home Office said over 105,000 offenders had been tagged since the scheme began and the reoffending rate remains below 3%.
Real Story: Tagging criminals is on BBC ONE, Monday 25 April at 1930 BST.