By March 2026, officials were checking 8,900 cases, covering almost one in four people recorded as requiring a tag. Some cases may simply have been left open by mistake. Other people had never been tagged.
The Ministry of Justice could not give a firm total for either group.
This is the service ministers intend to expand as they try to ease overcrowding in prisons.
The National Audit Office found delays in fitting tags and reporting possible breaches. It said the problems were wasting public money and putting public protection at risk.
Serco took over responsibility for fitting tags and monitoring offenders in May 2024. Within months, the number of outstanding visits to fit, inspect or remove equipment had climbed to 7,000.
By November 2024, the backlog had fallen below 400. That did not mean everyone was getting a tag.
Serco recorded 95 per cent of fitting visits as being made on time in February 2026. But after two visits, only 62 per cent had ended with a tag being fitted.
There could be another long wait when a possible breach occurred. Under the contract, Serco has between 29 and 53 hours to send the details to officials.
Police and probation teams then have to deal with it. They were not always given enough information and did not always have enough staff to respond quickly.
Nearly half the alerts with a recorded outcome ended without further action. During visits, auditors saw serious and minor breaches being dealt with in much the same way.
Ministers are pressing ahead with a much bigger tagging programme.
There were about 13,400 people under electronic monitoring in January 2021. By March 2026, the total had reached 28,700.
The government expects to add as many as 22,000 people a year from 2027. It has put aside up to £175 million for the expansion over three years.
Some of that money will support trials involving domestic abusers. A proximity tag is supposed to warn the authorities when an offender gets too close to the victim.
That warning offers little protection if the equipment has not been fitted or nobody is available to respond. Probation services were already about 2,200 staff short in March.
Tagging allows offenders to remain under restrictions after leaving prison. For victims, it may also appear to offer protection that the authorities cannot always provide.
The government has yet to publish a reliable total for people going without a tag when they should have one. It has not shown that police and probation teams can cope with the extra work either.
Despite that, another 22,000 cases a year are being planned.
Source: National Audit Office, “Electronic monitoring: improving resilience to meet increasing demand”, 10 July 2026.