Justice Minister Jim Wallace has defended plans to build three new private prisons in Scotland.

Appearing before the Scottish Parliament's Justice 1 committee on Thursday, Mr Wallace said it was the best way to modernise the prison system.

He justified the move by saying Scottish Executive research suggested the prison population could rise to 8,500 within the next 10 years.

But one committee member queried the need for extra prisons, and said non-custodial sentences could ease the prison population.

Jim Wallace: "Mark of faith"

The country's prison population has hit its highest level on record within the last few days.

There are currently 6,604 prisoners being held in state prisons north of the border with the number expected to rise.

Following the Prison Estates Review, the Scottish Executive announced controversial plans to build three private prisons and close those at Low Moss and Peterhead.

Mr Wallace said the move could save the executive �700m - a claim that has been hotly disputed in some quarters.

"These figures are based on long-term trends and don't take account of penal reform," he told the committee.

"There is no exact science in this but I don't think that should deter us from pursuing wider alternatives to custody, nor in trying to be responsible in modernising our prison estate."

'Public purse'

The minister maintained that building the new prisons in the public rather than the private sector was the correct course to take.

"With the private option we do not actually start paying until its delivered and until it is there to take the prisoners.

"Therefore the building time and the possibility of any delays is not something that would count against the public purse.

"In that way delays would count if it was a public option."

However, Labour member Maureen Macmillan questioned the need for more prisons.

She said greater use of non-custodial sentences could ease the prison population by about 1,500 each year.

The executive's move has caused controversy

But Mr Wallace argued this would not address the need to modernise the prison system and ending the practice of 'slopping out'.

Scottish Prison Service (SPS) chief executive Tony Cameron told MSPs that private prisons were half the cost because of "restrictive practices" within the public sector.

"The board is determined to seek increasing efficiency savings, but there will be limits to what we can do within the public sector," he said.

"But within those limits, we will continue as best we can."