A political war of words has erupted over the performance and viability of Scotland's only private prison.

Scottish National Party Leader John Swinney said that levels of vandalism, arson and possession of unauthorised substances made Kilmarnock the worst jail north of the border.

He said statistics collated from a series of Scottish Executive parliamentary answers illustrated that the prison was underperforming.

But the Scottish Executive dismissed the SNP claim as "simply bizarre" and said that the jail set a fine example to other penal institutions.

John Swinney: "Crazy plan"

Mr Swinney said he intends to write to Scotland's chief inspector of prisons and urge him to conduct an emergency inspection at the jail.

He said the private jail had 3,634 disciplinary offences in 2001-2 compared to 1,262 offences at HMP Edinburgh and 1,738 at HMP Barlinnie.

"These figures reveal a picture of Kilmarnock that makes it easily the worst prison in Scotland," Mr Swinney said.

Claim dismissed

"It has a disciplinary record that is staggeringly poor, with prisoners wandering through its halls apparently without a care in the world."

He added: "It is little wonder that it has the worst arson and vandalism record in Scotland as a result.

"Despite this, ministers remain intent on going ahead with their plan to build yet more private prisons.

"Their own statistics tell them that private prisons don't work yet they are so obsessed with privatisation that they cannot accept that it is time to dump their crazy plan."

Dr Richard Simpson said the SNP got it wrong

However, the executive was quick to dismiss the nationalists' claim as inaccurate.

Justice Minister Jim Wallace said Mr Swinney's conclusions were "simply bizarre" and that the private jail's performance was an example to other prisons in Scotland.

And his deputy, Dr Richard Simpson, said the SNP had misinterpreted the data.

He said: "If they (the SNP) had read the HM Inspectors' prisons report then they would have seen that Kilmarnock Prison has a number of pieces of best practice across the whole prisons' estate in Scotland.

"No prison is perfect but Kilmarnock is certainly not the worst."

The SNP in July said parliamentary answers suggested that the executive had tried to cover up subsidies to the prison.

Mr Swinney said at the time the subsidies amounted to �690,698, which was almost 70% of the estimated �1m profit made by Kilmarnock Prison Services in the past two years.