Scotland will need seven new prisons if current policies are not changed, according to a confidential report which has been sent to the new Justice Secretary.

The dossier, which is expected to inform decisions under the SNP-led executive, recommends radical changes to the criminal justice system to avert a dramatic increase in the prison population which would cost taxpayers an additional £250m a year.

Proposals include abolishing prison sentences of six months or less, in line with the SNP's manifesto; ending the practice of children being sent to jail, cutting the number of female inmates and capping the remand population.

The report, by Professor Alex Spencer, former head of rehabilitation at the Scottish Prison Service (SPS), has already been seen by Kenny MacAskill, the Justice Secretary.

Mr MacAskill will today start to outline his priorities for reform of the criminal justice system in his first speech in the role to parliament.

He is expected to emphasise the need to shift the balance towards the prevention of crime rather than cure, the importance of community punishment and the need to revitalise non-custodial sentences.

Mr Spencer says that, based on current prison service projections and a warning that the Custodial Sentencing and Weapons Bill, which has just gone through parliament, could mean an additional 1400 people being locked up, the total prison population "could well reach 11,900 and require seven new prisons" by 2030.

Likely sites for the extra jails, he suggests, would be Glasgow, Dundee, Dunfermline, Paisley, Hamilton, Cumbernauld, and Cambuslang.

This would also see the cost of prisons for taxpayers rise by £200-250m a year, with a bill of over £6bn extra over the following 25 years. Mr Spencer says: "Scottish society needs to stop thinking that prison can cure all the ills of society and instead to use some of the money we spend on prisons to improve and change lives in the community.

"The question we have to ask ourselves is whether we really want to pay this huge amount primarily to make things worse? It would be better value for money and for the community, victims and offenders to invest these huge sums of money to improve society, reduce reoffending and cut the prison population."

Earlier this year, The Herald revealed that ministers had given the go-ahead for a 700-cell prison at Low Moss in Bishopbriggs. The previous executive also agreed to a private 700-cell prison being built at Addiewell, West Lothian, and warned that the sentencing bill could make a third new prison necessary.

However, depending on the decisions of the new Justice Secretary, the future of the bill and the private prisons may now be in doubt.

Scotland proportionally has one of the highest prison populations in Western Europe. Official figures show that, of the 16,511 people given a custodial sentence in 2005, some 83% were given less than six months. Last year, more than 6000 people were sent to prison for fine default. Each place costs about £40,000 a year.

To avoid further dramatic increases in the prisoner population, Mr Spencer says the service should be restructured into two tiers, with a national system responsible for looking after the long-term convicted population, which currently sits at about 2900.

Lower level offenders would be dealt with through Community Justice Authorities (CJAs), community courts and localised prisons, and those given less than six months should serve a non-custodial sentence.

He suggests that CJAs, which are already in place, could be given control of the budget and the decision as to whether to spend their money on sending an individual to prison, or to a cheaper community-based option.

Community prisons, originally mooted by Lord Woolf's report after the riots in 1990 in Strangeways, Manchester, could be run locally rather than being governed by a central body, as is currently the situation.

The remand population should be capped and, if this figure is exceeded, those posing least risk should be released and then possibly tagged.

Mr Spencer's report was commissioned by the Scottish Consortium on Crime and Criminal Justice, the umbrella organisation for groups including the Howard League and Sacro.

His emphasis on community sentences is thought to chime well with the Justice Secretary, who is understood to be keen to ensure community sentences are better used and that the public can see the results.

A spokesman for Mr MacAskill said: "Our clear aim is to prevent and to deter crimes. That includes address the underlying causes of crime such as poverty, drug and alcohol abuse, particularly in our most deprived communities.

But those who do offend will face the consequences of their actions.

"The new government is making clear that prison should be for serious and dangerous offenders - not the flotsam and jetsam of society.

"So we need to shift the balance, with less serious offenders who are currently cluttering our prisons sentenced to community punishments instead.

"Government doesn't have all the answers to this. The Justice Secretary is open to ideas and keen to stimulate a wider debate across the political spectrum, professional groups and the public. He welcomes the consortium's early contribution and hopes it will stimulate others to come forward with their own views." The suggestions Abolish sentences of six months or less. Stop sending children to prison. Cut number of female prison inmates. Cap number of people on remand. Restructure system into two tier system of national and local prisons. Bring in community courts and community sentences.