THE estimated £17 million annual cost of an SNP plan to keep serious criminals in jail for longer will be paid for by reducing the number of small-time offenders put behind bars, Scotland's Justice Secretary has suggested.

Michael Matheson launched a defence of the Scottish Government's proposal to overhaul release arrangements for those sentenced to four years or longer, which would see serious offenders no longer automatically set free on licence two thirds of the way in to their sentence.

The proposals have been criticised by some experts, who have said the policy is at odds with the SNP's drive to reduce the prison population while concern has been raised that a compulsory six month period of community supervision at the end of sentences is insufficient to reintegrate offenders into society.

However, Mr Matheson, appearing before MSPs sitting on Holyrood's justice committee, insisted that the proposals were "proportionate" and said allowing the parole board more power to determine release dates would lead to a more effective system.

He said he had been "surprised" at criticism from the Howard League for Penal Reform, after the respected body said the Control of Release (Scotland) Bill, which by the Government's own estimation would increase the average prison population by 370 by 2030, was "at odds" with a pledge to reduce the number of people serving time.

Mr Matheson, who was appointed by Nicola Sturgeon to replace Kenny MacAskill in November, said: "There is far too much of our resource in the criminal justice system caught up with dealing with short term offenders who are in and out of our prisons on a constant basis.

"If you want to free up resource within our prisons to be much more effective in dealing with long-term offenders - those that post the greatest risk to our communities - we need to make sure that we are being much more intelligent about how we use our prison estate."

On the projected costs of the changes, which are projected to rise to £17m per year by 2030, he added: "It would be overly simplistic just to think of this as an additional cost that we have to put additional money in to meet. With the other changes that we take forward in the system, I believe that we'll free up some of that resource that can be better used, not only to deliver programmes for long term offenders but to deliver more effective community disposals as well. I was surprised by the Howard League's comments... you have to look at it in the whole rather than isolation."

However, while the Scottish Government has a long-standing opposition to short term jail sentences, it has so far struggled to reduce the numbers handed out in courtrooms.

In 2010, Mr MacAskill vowed to "end the free bed and board culture" when the SNP brought in controversial legislation which included a presumption against brief prison sentences. But in the first full year that the law was in force, 28 per cent of all offenders jailed received sentences of less than three months. The figure then rose in 2012-13, to 29 per cent, and remained at the same level the following year.

Academics who have previously criticised the Government's proposals before amendments to the Bill were made in response to concerns are due to appear before the Justice Committee today.

Ahead of the session Cyrus Tata, Professor of Law and Criminal Justice at the University of Strathclyde, called on the SNP to be transparent about what is now being proposed after it initially pledged to end automatic release for serious offenders, but then scaled back its proposals by saying they would instead be set free on licence six months early.

The change came in response to fears over 'cold release', meaning that under the Government's original plans, dangerous criminals would have been released from jail with no supervision or support increasing their likelihood of reoffending.

The alteration has led to the Scottish Government amending the title of its Bill from stating it will "end the right" to automatic early release for some prisoners to stating that it will merely amend the rules around the existing system.

Defending the move, Mr Matheson said: "The principle reason for the Bill itself was to end that automatic right to early release at two-thirds of the sentence and that's what the Bill achieves. What we are doing is making provision for that mandatory period of supervision to help support reintegration into the community."