Reliance has been criticised for its performance

Joseph Finnan, 20, went missing in Hamilton on Monday where he was due to make two separate court appearances.

Reliance confirmed he had been mistakenly released and the Scottish Prison Service said an investigation was under way.

The Scottish Executive has been under fire over a number of similar errors.

As a result of previous mistaken releases, the roll-out of Reliance's contract across Scotland has been put on hold by the Justice Minister, Cathy Jamieson.

Contract deadline

Scotland's Information Commissioner Kevin Dunion has given her until this Friday to publish the terms of the contract between the Scottish Executive and Reliance.

In a statement, Reliance confirmed that Mr Finnan, from Uddingston in Lanarkshire, had been released in error while in the firm's custody.

Jack McConnell has defended the Justice Minister, Cathy Jamieson

He had been arrested for the non-payment of a fine and appeared at Hamilton District Court where he paid the fine.

Mr Finnan was then due to be taken by Reliance to face an unconnected charge of assault at the town's sheriff court.

Reliance's Campbell O'Connell said: "It's difficult to give a guarantee but we are certainly taking all steps to avoid such occurrences.

"I think it's important that the public and the legal system is reassured that we will provide a first class service."

Opposition MSPs have strongly criticised Ms Jamieson and the executive for continuing the contract with Reliance.

She has resisted demands for her resignation and has been supported by the First Minister, Jack McConnell.

BBC Scotland home affairs correspondent Reevel Alderson said the prison service was known to be frustrated by the way such incidents were being treated and that releases of prisoners in error had always taken place.

Sources said the only difference now was that the system was transparent and when mistakes happened they would be investigated and errors put right.