Prisoners could be put in portable cell blocks to ease overcrowding under SNP plans.

Justice Secretary Neil Gray has ordered the Scottish Prison Service to ‘exhaust all options’ in the search for more spaces, including ‘temporary modular accommodation’.

The long-delayed £209million HMP Highland in Inverness is due to be completed in spring 2027 and the £1billion HMP Glasgow, which replaces Barlinnie, in 2028.

But Mr Gray said they wouldn’t be enough to cope long-term with a rising prison population, and new facilities were needed soon, as well as curbs on short term sentences and serious offenders being let out early, to reduce numbers.

He told MSPs: ‘I have instructed the Scottish Prison Service to exhaust all options for further maximisation of the existing estate, and to set out an approach for how we can further increase the number of places.

‘This will include the affordability and deliverability of additional capacity, including consideration of temporary modular accommodation and further house blocks at existing prisons. I expect business cases on these soon.’

Portable cells typically have a private ensuite with a shower and toilet, a desk, chair and bed.

Justice campaigner Kenny Stewart, whose 17-year-old daughter Michelle was murdered in 2008 by her former partner, said: ‘Putting prisoners in these “modular” building sounds like a bad idea – how structurally secure will they be?

‘Is it beyond the powers of the SNP to build actual prisons that don’t cost £1billion, like the replacement prison for Barlinnie in Glasgow?’

Dr Hannah Graham, a criminologist at Stirling University, said: ‘Considering building a few more house blocks in existing prisons is a band-aid measure amid pressured conditions.

‘It doesn’t solve longstanding and increasingly acute problems - it simply tries to cope with rising numbers.

‘Other proposals announced by Scottish Government are more likely to address the contributing factors to Scotland’s high prison population and overcrowding.’

Scotland has no ‘modular’ cells at present, but prefabricated ‘Rapid Deployment Cells’ are used south of the Border.

The self-contained units, which can be stacked on top of each other, have a 15-year life span and are used for low-risk and female prisoners on existing prison sites.

Around 800 have been built in England, with another 1,000 planned.

An evaluation last year found they were liked by both inmates and staff.

Prison officers felt they reduced violence and self-harm, while offering them could be used to incentivise good behaviour.

Phil Fairlie, of the Prison Officers’ Association, said: ‘Talk of providing extra accommodation is welcome and in the current situation, desperately needed if we are to give staff the breathing space they need to keep themselves and the prisoner population safe and secure.’

But he warned: ‘Those extra spaces must come with the subsequent staff increase needed too, and we will watch with interest to see what form those accommodation spaces take.

‘Ideally, they will be as a short to medium term step towards a longer-term outcome of fewer people being housed in our prisons in the future.’

The Scottish Prison Service said security would be the priority with all accommodation.

A spokesman said: ‘We need the right level of capacity to safely and securely manage our population and, working alongside Scottish Government, will assess all available options.’