The former home secretary responsible for introducing indeterminate prison sentences said he "regrets very much" the problems caused by the way they were implemented.
Offenders given the sentences are not released until they are assessed as posing no danger to the public, and David Blunkett said the Labour government was not "effective enough" in making the necessary resources available to provide rehabilitation courses for those jailed under the measures.
Indeterminate sentences for public protection (IPPs) were abolished in 2012 but thousands that had already been handed down remain in force.
T he Criminal Justice Act of 2003 introduced IPPs for serious sexual and violent offenders in England and Wales, but because the provisions were quite broad judges began using them for more offenders than expected.
Mr Blunkett told BBC2's Newsnight the government was not clear enough in setting out sentencing criteria for judges.
" The consequence of bringing that Act in has led, in some cases, to an injustice and I regret that," he told the BBC.
Newsnight highlighted the case of Richard Huxley, who remains in prison more than eight years after being given an IPP with a minimum tariff of 17 months for assault and attempted robbery.
Mr Blunkett said: "I would say that this is an injustice. I would say that the original intention had nothing to do with circumstances where people would be held way beyond the normal tariff, in a situation where in some instances they have not been able to take the necessary course and demonstrate the rehabilitation necessary for changing their behaviour."
He added: "I implemented what I believed was necessary to safeguard the public but - and I regret very much that we were not clearer in terms of the criteria laid down, tougher in terms of saying what the judges should or should not do, and we were not effective enough in putting in the necessary resources to ensure that the rehabilitation courses were available - we certainly got the implementation wrong but the intention was in my view correct."
Huxley, who was jailed at 18, has failed a drugs test during his time in prison but his family argues that in eight years he has not been violent.
His mother, Wendy Huxley, from Ellesmere Port near Liverpool, said: "He's lost so many years, it's wrong. He's in there and forgotten about, basically."
Prisons minister Jeremy Wright said: " The release of prisoners serving indeterminate sentences is entirely a matter for the independent Parole Board, who must be satisfied that an offender can be managed safely in the community.
"If prisoners pose too high a risk of harm to the public, they can be kept in custody for the rest of their lives.
"The IPP scheme was complex and widely criticised, which is why the Government replaced it with a new regime of tough, determinate sentences, alongside life sentences for the most serious offenders.
"However we have no intention of retrospectively altering lawfully imposed IPP sentences - they were handed down for the most severe crimes to ensure public protection."