A riot at a young offenders institution last month was the worst disorder in one of Scotland's jails in a decade, the prison service has said.

Prison officers from four other jails were called in to restore control during the incident at HMP & YOI Polmont in September, which involved about 30 inmates.

Linda Pollock, chief executive of the Scottish Prison Service (SPS), said it was an example of "bubbling tension" caused by increasing overcrowding within the estate.

It comes after the Scottish government released projections estimating the prison population could rise to 9,450 next year – 1,650 over the current design capacity.

Justice Secretary Neil Gray said the government is considering reforms to "achieve a sustainable prison population".

Pollock said the predicted increase would make it impossible to safely accommodate prisoners and protect the welfare of staff. Several prison officers suffered minor injuries in the riot at Polmont on 26 September.

Images released by the SPS to BBC Scotland News of the aftermath show panelling damaged and torn out, furniture broken and strewn across the hall and a glass window cracked and shattered. Police Scotland is investigating the incident.

Pollock said the outbreak of disorder was "not unexpected" given the rising strain placed on prisons by the sharp increase of those held in custody.

"We did have disorder in Polmont, there was destruction there and we had to take control of the situation," she said.

"People in custody are becoming frustrated by the overcrowding conditions, the impact that has for them and restricting the opportunities they have for rehabilitation."

Pollock added: "We are seeing bubbling tension across the estate. We pushed our prisons to the brink and now we are at the brink and action needs to be taken." Pollock highlighted the recent incident of interruption to the water supply at HMP Barlinnie as an example of the "fragility" of the Scottish prison estate.

She also warned that Scottish prisons were at serious risk of a return to levels of disorder "last seen in the 1980s" if conditions did not improve.

Two major riots, at HMP Barlinnie and HMP Peterhead took place in 1987 as a result of poor conditions inside the jails - including 'slopping out', the practice of using buckets in cells as toilets. On both occasions, prison officers were taken hostage.

Pollock said the current system was "close to breaking point" and that action was needed to ensure it "does not break". About 8,500 inmates are currently held in Scotland's jails.

In May, a new record high for the prison population was set, with 8,603 inmates being held in a system designed to accommodate 7,805.

Since June 2024, about 1,600 prisoners have been freed early to ease pressure on the system, through waves of emergency releases and changes to the rules.

The automatic release point for some short-term prisoners was lowered last year from 50% to 40% and fell again in May to 30%. The change did not apply to prisoners convicted of sex crimes or domestic abuse.

However, the number of inmates has remained well above the designed capacity, with about 700 more long-term prisoners in jail now than two years ago. That has been partly driven by an increase in convictions over serious organised crime and historic sexual offences.

Pollock said special measures including two prisoners sharing cells intended for one had been introduced.

A new £209m jail, HMP Highland, is due for completion later this year to replace the 124-year-old HMP Inverness with capacity for 200 inmates.

Meanwhile, HMP Glasgow, a replacement for the 143-year-old Barlinnie, is expected to open in 2028 at a cost of almost £1bn.

The 1,344-capacity facility was originally scheduled to be ready in 2025 and cost estimates in 2022 put the final bill at £400m.

Phil Fairlie, assistant secretary of the Prison Officers' Association (POA), described the projected increase as "shocking and depressing".

He said the prison service would reach a point of "saying the doors are shut" to new inmates should the projections be accurate.

Fairlie said: "From the outset the POA said that the early release scheme would only provide temporary relief and that the spaces freed up would immediately be filled by newly convicted prisoners.

"We need urgent action to address the lack of prison accommodation and the understaffing of the prison estate, and vastly improved support for our members who are on the frontline of this crisis and working under intolerable pressure." In June, the Scottish government launched a consultation on permanent new measures to address issues with overcrowding.

One proposal would change the definition of long-term prisoners from those serving four years or more, to those serving five years or more. That would push some prisoners into the category where they would be freed automatically after serving less than a third of their sentence.

Another proposal would see the automatic release point for long-term prisoners - including killers, rapists and child abusers - changed to two-thirds of their jail term At present, if they're not granted parole, they are freed when they have six months left to serve.

Under another suggestion, judges would be encouraged to avoid issuing prison sentences of less than two years, up from the current presumption against sentences of 12 months or less.

The proposals followed a series of recommendations laid out in a report by the Scottish Sentencing and Penal Policy Commission and would require the backing of MSPs to come into force.

Gray said the Scottish government was increasing investment in community justice, but acknowledged further action was needed.

"To ease the pressure, we have maximised capacity across the prison estate by 640 over the past three years and we are building two new prisons," he said.

"Alongside expansion of the prison estate, the reforms will strike the right balance across custodial sentences, community sentencing and the need to robustly protect victims and the public." Scottish Labour's justice spokesperson, Pauline McNeill, said the situation was "completely unsustainable".

She added: "The justice secretary must come forward with a comprehensive, long-term plan for tackling these issues, including evaluating proper alternatives to custody and supporting hardworking prison staff." Scottish Conservative justice spokesman, Stephen Kerr, added: "The SNP's only answer is to water down the consequences for criminals after two decades of failing to build the prisons Scotland needs.

"Enough is enough. The SNP must stop putting the needs of offenders ahead of public safety."