Natalie Beal, chair of the Prison Governors’ Association Scotland (PGAS), said staff were under “significant strain” and called for “urgent mitigation”.

Measures could include safe emergency release and greater use of electronic monitoring.

Justice Secretary Angela Constance had previously said that the pressure was “a great cause of concern” and that she was taking action.

In a letter to Holyrood’s Criminal Justice Committee, Ms Beal said PGAS wished “to formally raise our significant concerns around prison population pressures and the predicted trajectory of further increase in population numbers”.

She said: “We are equally concerned at the increasing complexity of Scotland’s prison population and the ability to appropriately locate individuals, given the pressures on space. If we continue this path, we are at best providing humane containment, albeit even this may be at a stretch, with many prisons already operating restricted regimes, resulting in the provision of little or no positive rehabilitative work.

“The pressures on staff are already evident, with many prisons facing high sick absence on a daily basis. Increased population numbers will also require increased staffing. Our membership and all staff in prisons already do a very challenging job, but are currently under significant strain.”

A Scottish government spokesperson said: “While prison is needed for those who pose a risk to public safety, we recognise the challenges the prison service is facing, in particular cost increases linked to inflation and the increase in population and we are taking action to address that.”