A new electronic monitoring programme for criminals is running at least five years behind schedule after a catalogue of failings by the Ministry of Justice (MoJ), a spending watchdog has warned.
The assessment by the National Audit Office (NAO) painted a chaotic picture of the department's handling of the project, which was first launched six years ago.
It concluded that the MoJ failed to achieve value for money in its management of the scheme, which will have cost an estimated £130 million by 2024-25. The service itself is expected to cost £470 million between 2017-18 and 2024-25.
Since 1999, the Government has used contracted-out electronic monitoring, or "tagging", services for the sentencing and supervision of offenders.
The regime is used by police, courts or probation services to monitor offenders' locations and compliance with home curfews.
In 2011, the MoJ launched a programme to develop a new "world-leading" ankle tag that combined radio frequency and GPS technology.
It set out to procure the service using a new "tower" delivery model, which incorporated contracts with four separate suppliers who would provide four different elements of the service, with their work pulled together by a contracted integrator.
The NAO found the Ministry did not do enough to establish the case for location monitoring using GPS.
There is still limited evidence on the effectiveness of electronic monitoring in the UK, the report said, with most experience of location monitoring based on small numbers of volunteers rather than mainstream offenders.
"The Ministry assumed there would be high demand for location monitoring from those who sentence offenders but did not run a pilot to test this before launching the programme," the review said.
"It also did not understand the potential financial costs and benefits of expanding location monitoring."
The MoJ is "only now" running location monitoring pilots to test how the use of a GPS tag might affect the behaviour of offenders, the report noted.
It also found:
:: The Ministry's bespoke requirements for world-leading tags proved too ambitious, with 900 prescriptive requirements evolving over time.
:: The planned time scale for the programme was unachievable, and the new tags are now expected to be deployed from the end of next year - five years later than planned.
:: Original projections suggested that between 160,000 and 220,000 offenders would be tagged in 2016/17 - when in fact the number under the existing programme is expected to be under 65,000.
::The MoJ's governance arrangements were weak, resulting in slow decision-making and allowing internal disagreements to persist.
Amyas Morse, head of the NAO, said: "The case for a huge expansion of electronic monitoring using GPS was unproven, but the Ministry of Justice pursued an overly ambitious and high-risk strategy anyway. Ultimately, it has not delivered.
"After abandoning its original plans, the Ministry's new service will now, ironically, be much closer to its existing one. Even if it launches in 2018, it will still be five years late."
The NAO said that, following internal and external reviews, the department has taken action to address many of the issues, making changes to set a more achievable requirement and strengthen programme management.
Mr Morse said: "The Ministry has learnt costly lessons from its failings but significant risks still remain."
Liberal Democrat home affairs spokesman Sir Ed Davey said:"The disgraceful waste of public money throughout this saga has been appalling. It has been bungle after bungle and now we learn that very little will change after all."
A Ministry of Justice spokeswoman said: "As the National Audit Office makes clear, there were challenges in the delivery of the electronic monitoring programme between 2010 and 2015.
"As a direct result, we fundamentally changed our approach in 2015, expanding and strengthening our commercial teams and bringing responsibility for oversight of the programme in-house.
"We are now in a strong position to continue improving confidence in the new service and providing better value for money for the taxpayer."