Another prisoner has escaped from custody while being transported to court by Reliance Security Services.
The inmate escaped through the roof of a van
David Duffy was due to face assault and disorder charges and was on his way to Glasgow Sheriff Court when he escaped through the roof of an escort van.
The Scottish Prison Service (SPS) said it was investigating the incident.
Earlier this month it was revealed that up to 17 people may have been released in error since Reliance began prisoner escorts in the west of Scotland.
However, managing director Tom Riall told a Scottish parliamentary committee that six of those releases could be blamed to some extent on Reliance.
Police launched a manhunt after the latest blunder.
Mr Riall confirmed that a 19-year-old prisoner had escaped from one of the company's vehicles, through an emergency hatch in the roof, but said the incident should not be taken out of context.
He said: "This is only the first escape we've had from one of our vehicles.
"We've moved more than 13,000 prisoners and have only incurred two escapes."
Mr Riall said there was always a danger of escapes when moving large numbers of people.
"That was the case long before Reliance took over. It's our job to investigate what has happened," he added.
Reliance would continue to provide a safe and secure service, he added.
Justice Minister Cathy Jamieson, who has come under fire from opposition parties over the problems with Reliance, expressed concern about the new lapse in security.
'Clearly a problem'
She said: "I am clearly concerned to hear about this morning's escape from a Reliance vehicle.
"SPS are investigating the circumstances into the incident and I expect them, with Reliance, to take appropriate action when those circumstances are clear.
"Escapes are rare but, sadly, they do happen. However, this is particularly disappointing since Reliance's systems for transferring prisoners are designed to be even more secure that the procedures they have replaced.
"Even from initial information, it would appear clear that this is a problem within the escorting service and not with any other part of the court system."
The minister pointed out that a system was in place to impose financial penalties on Reliance for escapes.
The Scottish Conservatives called on the justice minister to resign over the affair.
Tory justice spokeswoman, Annabel Goldie, said: "Just how many escapes have to take place before Cathy Jamieson does the right thing and resigns?
"Whoever is to blame each time, whether it is Reliance or the prison service, the minister cannot duck responsibility and pass the buck.
"If she will not resign, then Jack McConnell must sack her."
The Scottish National Party said the Scottish Executive should cancel Reliance's contract.
SNP justice spokeswoman, Nicola Sturgeon, said "the fiasco gets more farcical by the week".
"The Reliance contract is not worth the paper that it is written on, public safety is being compromised and the justice minister must put an end to the chaos."