Fears over offenders wrongly returned to jail because of faulty tag straps
Equipment used to monitor dozens of offenders or suspects in the community is faulty, it has emerged.
Straps used to secure electronic monitoring tags could incorrectly trigger an alert suggesting they have been tampered with.
Tests indicate that more than 100 of the straps are faulty, while a small number of individuals may have been wrongly placed behind bars.
The Liberal Democrats claimed it was a "colossal blunder", while the Government insisted there was no risk to the public.
Justice Minister Sam Gyimah said there was a "small chance" that some "enforcement action" might have been taken against an offender or suspect in response to a false report of tampering.
"It does not mean an individual will have been automatically sent to custody," he said.
"A single tamper alert without any additional evidence of an escalation of risk is likely to result in an alternative outcome, such as a warning letter.
"So it is unlikely that a first tamper on its own will result in an offender being recalled."
Officials said that if any individuals had been returned or sent to custody, the number was likely to be small.
MoJ staff notified Electronic Monitoring Services (EMS), which provides the electronic monitoring service, of an increase in the number of alerts raised when equipment worn by an offender or suspect is tampered with.
This was investigated by EMS and G4S, which supplies straps and electronic tags used to monitor offenders and suspects who have a curfew as part of their sentence or bail.
At the end of last month, G4S informed the ministry of an issue with faulty straps.
Mr Gyimah said: "We understand that the number of affected straps is small. Only straps that have entered the system since October 2016 are affected.
"This is the point at which the batch of potentially faulty straps entered circulation. G4S has been testing straps. That testing indicates that around 1% (115) of the 11,500 straps in use today are faulty.
"If no tampering with the tag has been registered, they have operated as normal. Where a strap is faulty, however, there is a risk that it could incorrectly register that somebody has tampered with it."
He stressed that the monitoring functions of the tags themselves are not affected and the security features within them have been working correctly.
"Therefore, there has been no risk to the public," the minister added.
"G4S has introduced further quality checks with the strap manufacturer to ensure that no more faulty straps enter the supply chain. The taxpayer will bear no cost for the faults."
Liberal Democrat leader Tim Farron called for an urgent review, saying: "People may have been wrongly sent back to prison because of this Government's sheer ineptitude."
David Byrne, managing director for G4S Monitoring Technologies, said it identified a "manufacturing defect" in approximately one per cent of two specific batches of tag strap clips.
He said: "This fault posed no risk to the public and has now been rectified with our supplier and the new clip production process has been tested and independently certified by the Ministry of Justice's approved test laboratory."
Work to replace all potentially faulty strips is under way, while courts have been informed of the problem.