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A computer glitch has resulted in the wrong risk level being attributed to hundreds of Scottish offenders, the country's justice secretary has said.

The risk scores are used in sentencing and prison release decisions.

But an IT error meant that the score did not change when new information was entered into the system.

The Conservatives said the "horrendous blunder" could have put the public at risk by allowing prisoners to be released when it was not safe to do so.

Justice Secretary Keith Brown said feedback from 150 users of the system had not identified any public protection issues so far.

More than 1,000 cases are being reviewed, and the computer system has been shut down in favour of a paper-based alternative in the meantime.

Mr Brown told MSPs a problem had been flagged up with the level of service and case management system (LS/CMI) used by the criminal justice system, which helps set risk levels for use in sentencing and prison release decisions.

Offenders are given a numerical risk score in the system, but Mr Brown said cases had been discovered where the score was incorrect as it had not changed when new information was entered.

The justice secretary said 1,317 assessments had been made where scores in the system "did not match" the final risk level calculated - 1,032 of them "closed" or completed cases, dating back to 2012.

In 537 of these cases, social work teams applied an over-ride, based on professional judgement - leaving 495 which "appear to contain a risk level affected by the system error".

However the cases where an over-ride was applied will all need to be reviewed to determine if this superseded any error.

And the 285 open cases where errors have been found are also under review, with priority given to those where prisoners are being considered for imminent release.

Mr Brown also said a second problem had been discovered while officials were investigating the errors, relating to another risk scoring system relating to alcohol and drug use.

He said the extent of this issue was not known, but that it could also affect risk scores.

The justice secretary stressed that in the cases reviewed so far, no concerns had been expressed by social workers in the community or in prisons.

And he said professionals involved in the justice system were experienced in managing risk, and "never just follow what is displayed on the screen".

But opposition parties said hundreds of criminals could potentially have been wrongly released from prison.

Scottish Conservative justice spokesman Jamie Greene said: "This is a grave error on the justice secretary's watch no matter how he spins it.

"He claimed that the public have not been put at risk but the reality is he doesn't know yet. He has absolutely no idea if this mistake has resulted in harm to the public. That is shocking and unforgivable."

And Scottish Lib Dem MSP Liam McArthur said the situation was "profoundly concerning", adding: "Over 1,000 people could have either suffered longer sentences, or been released much sooner than they should have. There are no winners in this situation."