1 in 4 satellite-tagged criminals reoffends in weeks

Their offences included a rape and two more described as "very serious".

Other criminals simply ditched the equipment. One was on the run for more than 200 days.

The technology for David Blunkett's much-vaunted "prison without bars" scheme was found to be badly flawed; for instance it didn't work in the shadow of a tall building.

When it was launched in 2004, the then Home Secretary claimed the system could pinpoint an offender's position to within six feet.

Now the entire pilot, which has cost £3 million, looks certain to be ditched after the damning findings of an evaluation report.

Under the scheme, convicts wear an ankle tag and a separate portable tracking unit, which uses mobile phone technology, costing £42 a day to operate.

The unit relays the offender's movements, via satellite, to a control room run by private firms such as Premier or Serco.

The aim is to alert the authorities if the criminal should enter an "exclusion zone" - such as a paedophile entering a children's play area - if a curfew is missed or if the equipment is removed.

It was also supposed to "deter" people from reoffending, as they would know they would be recorded as being at the scene of any crime.

But six out of ten criminals placed on the scheme breached their conditions by ignoring curfews, entering so-called "exclusion zones" or re-offending.

In total, 26 per cent committed new crimes. Those on the scheme included burglars, robbers, sex attackers and violent thugs.

The vast majority were "high risk" criminals with lengthy records.

Scores were classed as "unlawfully at large", breaching their curfews or removing equipment. In one case, a criminal remained at large for 233 days.

Shadow Justice Secretary Nick Herbert said: "Public safety has been compromised by this disastrous experiment. High-risk offenders have committed very serious crimes when they should have been in custody.

"The Government is refusing to provide adequate prison capacity and instead wants to build trust in community sentences.

"That cannot happen when sex offenders and violent criminals are released early with inadequate supervision."

The report's author Stephen Shute, professor of criminal justice at Birmingham University, said:

"It was recognised that tracking units would have difficulty picking up signals when located within buildings and that, even when carried in the street, the presence of tall structures could impede or distort the signals that they were able to receive.

"It was also recognised that offenders who were determined to commit crime could forcibly remove their ankle tags or leave their tracking units behind."

A Government spokesman said last night: "The Ministry of Justice is considering the evaluation report, and the recommendations of a National Offender Management Service Working Group, on the future of satellite tracking of offenders in England and Wales."

Justice Minister David Hanson yesterday invited bids for the private finance contracts to build two 600-space jails, in south London and near Liverpool.

They will not open until 2010, despite the current overcrowding crisis.