The existing design capacity of Scotland’s prisons is 7,805, yet the total population stands at 8,391 – almost 600 more than the system was built to accommodate.

Despite two new facilities under construction, Scotland’s jails remain at record levels of overcrowding. Ministers are pursuing a third early release scheme to ease pressure, but projections suggest the additional capacity will fall short.

HMP Glasgow – which is set to replace the Victorian-era Barlinnie prison at a cost of up to £1 billion – is expected to add 357 spaces. HMP Highland, also under development, will increase capacity by 107.

Together, the two projects will raise the total capacity to 8,269 – still more than 100 places below the number of people currently in custody.

Scotland’s prisons also operate under an “operational capacity” threshold — the upper limit at which they can function effectively. That figure currently stands at 8,007, leaving the system over capacity by more than 300 inmates. It remains unclear by how much the new prisons will improve that figure.

Scottish Labour leader Anas Sarwar said the scale and cost of the expansion failed to address the underlying problem. “The SNP are ploughing ahead with their plans to spend over £1 billion on HMP Glasgow – the same as it cost to construct the five-star Atlantis Hotel in Dubai, one of the most luxurious hotels in the world,” he said.

“Yet these alarming figures show that despite the eye-watering cost of this new facility, it will not even be fit for purpose by the time it has been completed, with Scotland’s prison population already at a record high. That means taxpayers will be paying well over the odds for a prison that was supposed to cost £100 million, and which will not even fix the crisis in our prisons estate, with staff warning that they are at a crisis point.”

Mr Sarwar added: “This is a mess of the SNP’s own making. Scots are having to foot the bill for SNP incompetence, with ministers out of ideas for getting on top of this dire situation.”