Overcrowding in Scotland's jails could be eased by legalising hard drugs, a former chief inspector of prisons says.

Clive Fairweather, who held the post from 1994 to 2002, said that the prison population would drop if doctors prescribed heroin to addicts to stop them from committing crimes to feed their habit. He also said that more jails were needed and urged an extension of the tagging system to tackle the rising prison population.

Andrew McLellan, the present inspector, said in a BBC TV documentary last night that overcrowding in Scottish prisons was endangering public safety. It meant that short-term inmates received little or no rehabilitation and were more likely to reoffend.

Kenny MacAskill, the Scottish Justice Secretary, admitted that dealing with overcrowding was depriving prison staff of the opportunity to tackle the behaviour of the most serious - and dangerous - offenders.

Last year Scotland's prison population hit a record high of 7,497. Yesterday it stood at 7,284, which is 658 inmates more than the designated capacity.

Mr Fairweather, who previously favoured more community alternatives to sentencing as a means of lowering the prison population, told The Times that he had reassessed his opinions in the face of the unrelenting rise.

He announced his revised views during a talk to retired doctors this week at the Royal College of Physicans in Edinburgh. He said: “Reducing overcrowding in jails will require a combination of approaches, including more community sentencing, tagging for remand prisoners and building more jails. We have made a start on tackling the alcohol abuse that contributes to crime but we will also have to look at the whole business of drugs and whether they should be legalised.”

He suggested that GPs should be given the power to prescribe heroin to prevent addicts committing crimes to feed their habits. “My own belief is that we need to get back to where we were in the 1960s when, instead of jailing Sherlock Holmes and his opium supplier, Dr Watson, he would have been referred to a doctor.

“Medically I don't know if it is better to prescribe heroin over methadone, but I think it would be better for the safety of society. I have come to that conclusion reluctantly after watching what has happened over the past ten years.”

He added: “Scotland can't go on. We don't have the space to build all of the jails we would need if the increase continues, we don't have the money to build them, or the staff.”

Dr McLellan said that the prison system was failing the public, inmates and staff. “Overcrowded jails mean Scotland is less safe; for the sake of us all we need to make sure overcrowding is defeated,” he said.

Scotland's biggest prison, Barlinnie in Glasgow, has a population 50 per cent over capacity. But Dr McLellan said that more jails would do “almost nothing” to solve the problem.

In a BBC radio interview yesterday, Mr MacAskill conceded that inmates were entering prisons in “revolving door circumstances” that were damaging efforts to provide rehabilitation.

“The Prison Service in Scotland is excellent but if it is suffering with volume that it can hardly cope with then clearly they cannot address underlying matters,” he said. He added that ministers were hoping to tackle overcrowding through the prison commission, headed by Henry McLeish, the former First Minister. The extension of home detention curfews, in which non-violent prisoners nearing the end of their sentence are released early under supervision, and other alternatives presently being explored, would also help, he said.

He added: “In the longer term, we need to ensure we are working hard to develop a coherent penal policy that allows prisons to do their job with serious and violent offenders but diverts those offenders who do not present a risk to the public and for whom prison is manifestly not working.”