The prison and probation service does not know how many people ordered to wear an electronic tag are actually being monitored.
That is the central finding of a National Audit Office report published this month. As of March 2026, HM Prison and Probation Service was reviewing about 8,900 cases, roughly a quarter of everyone recorded as requiring a tag, to work out how many were going unmonitored. The Ministry of Justice says the real figure is lower, at around 5,450. The NAO’s own conclusion is blunter than either number: HMPPS “does not have an accurate understanding of the number of individuals that should be monitored and are not”, and that number “could be significant”.
Some of those cases will turn out to be nothing: records left open by mistake, people back in custody or tags awaiting fitting. But the 8,900 is a review still in progress, not a final tally, and the NAO says the true number of unmonitored people could be significant. There could be more.
The system currently tags around 29,000 people, more than double the number in 2021, at a direct cost of roughly £133 million a year, or about £4,900 per person. The point is to manage more offenders in the community and take some pressure off overcrowded prisons. More people are due to be tagged from September. By 2027, the government expects its early release plans and the new Sentencing Act to add up to 22,000 more people to the tagging system each year.
Ministers are preparing to expand a system that the NAO says is not working effectively.
Response to breaches is another problem. When probation and police staff do learn that a tag has been breached, nearly half of those breaches result in no further action. The NAO found that serious and minor breaches were often handled in the same way, and staff across the system did not agree on how each should be dealt with. Under some contracts, officials are not told about a breach until 29 to 53 hours after it happens. In the worst cases, more than two days pass before anyone is alerted.
Part of the problem lies with the contractor. Serco, which fits and monitors the tags, performed poorly between August 2024 and July 2025. People were not tagged on time and officials were not told about breaches promptly. The NAO says performance has since improved, but there is still an estimated shortage of around 2,200 probation staff. Those missing staff will somehow have to help monitor tens of thousands more people.
Gareth Davies heads the NAO. He said: “Electronic monitoring is central to managing pressures on prisons, but it is not working effectively, creating risks to public protection.”
The government disputes how bad things are. It does not dispute that there is a gap.