A firm responsible for tagging and tracking offenders has failed to monitor some criminals for weeks, a BBC undercover investigation has found.
The tags were removed by some offenders
Group Four Securicor (G4S), responsible for monitoring more than 1,000 people across the East Midlands, said it had failed in "a small number of cases".
A BBC Inside Out inquiry found several men described by G4S staff as sex offenders were among those unmonitored.
Five workers have been suspended and a Home Office investigation has started.
Ian Ridgley, chief operations officer for G4S Justice Services, said: "We recognise that the Home Office require us to work to a very high standard and of course we are sorry that in some minor number of instances we may not have operated to those standards."
A BBC undercover reporter spent four months working as a temporary employee at G4S where he secretly filmed workers and managers and kept a detailed diary.
He filmed G4S staff allegedly fabricating records of tagging installations to save money and meet government targets.
The BBC also learned tags were routinely removed from alleged offenders the night before their bail hearing on Home Office instruction.
One violent offender was filmed drinking after his tag was removed by G4S. A week later he was sent to prison for assaulting a former partner.
The policy of removing the tags before hearings is being reviewed by the government.
The Home Office has ordered a full audit of G4S records across the UK - and is reviewing its contract with the firm.
When an offender is tagged, G4S staff install a monitoring box at their home to keep track of their whereabouts.
A signal is sent from the box to G4S headquarters in Manchester where any breaches of probation are followed up.
The investigation discovered that failure of monitoring equipment could usually be traced to unreliable mobile phone technology used to send the signal to headquarters.
When contact was lost with a tagged offender, it sometimes took weeks for staff to visit the house to address the problem.
One employee told the BBC undercover reporter that the firm was not able to track the offenders if the technology failed.
A man convicted of Marian Bates' murder had taken off his tag
"We don't know what they're doing. They could have been in Barbados for a week while it is 'missing status' - we wouldn't know."
Staff told the reporter the monitoring boxes could not be relied on: "The kit is rubbish - it lets you down every time."
A G4S spokesman said its equipment is effective and has been tested by the Home Office and its scientific branch.
Although the firm admitted problems in some cases, it was not prepared to say exactly how many cases or what type of offenders it had not been monitoring.
But one offender said he had taken his tag off 10 times during curfews and was never contacted about it.
Another offender who disconnected his monitoring box for six days said: "Obviously they aren't doing their job properly - they should be resetting the box and putting me back on the tag."
A G4S employee was secretly recorded saying: "I've got three paedophiles walking around not being monitored."
Marian Bates, a jeweller from Nottingham, was killed in a robbery attempt at her shop in 2003.
When the gunman's accomplice Peter Williams was put on trial for her murder it emerged he should have been wearing a tag - but he had taken it off before the killing.
Mrs Bates' husband Victor said the Inside Out findings had confirmed his view that the tagging system is not fit for purpose.
The system was then run by a different company, Premier.
"This has just shown the silliness, the sheer incompetence of tagging.
"But if you're not confident that everything's being done to obviate the risks where do we go, who's next?"
Some of the equipment was faulty
Harry Fletcher, from the National Association of Probation Officers, which represents 9,000 probation officers in the UK, said he was not convinced tagging should be run by private companies.
"I knew that there were problems with tagging but what this programme shows it that the problems with tagging are immensely worse than I thought.
"We have questions of falsifying records, of dishonesty, of faulty equipment and given that this scheme's been around for the best part of 20 years it is both extremely surprising and very shocking."
Inside Out East Midlands is broadcast on Friday on BBC 1 at 7.30pm.