Calls have been made for HMP Greenock to be replaced as soon as possible after a leaking roof forced the closure of 40 cells.
The prison was built in 1907 and operates at only 75 per cent capacity because of the poor condition of its buildings. A replacement, however, is still a decade away, Wendy Sinclair-Gieben, HM chief inspector of prisons said.
She also said the Scottish government had a “real crisis on its hands” when it came to prisons.
She told Good Morning Scotland that while Greenock, which was the subject of a report, was a “very good wee prison” its condition was unacceptable.
She added: “There are two stark choices for government; one is to reduce the prison population so Greenock is not necessary and the other is to consider some form of building that can be achieved within a shorter time.
“I think the Scottish government has a real crisis on its hands with a rising prison population and an estate with so many prisons that are not fit for purpose.”
The report by HM Inspectorate of Prisons for Scotland (HMIPS) praised the jail for its response to the pandemic; it had no confirmed cases a year into the crisis.
The report said: “The primary concern is the ageing infrastructure of HMP Greenock and the unsuitable environment created by the fabric of the building.”
However, it described it as “ill-suited to a modern prison system and accordingly in urgent need of replacement”.
The report added: “A robust permanent solution must be found in the interim for a number of issues; to prevent further ingress of water into healthcare settings and accommodation areas to minimise the risk of transmission of infection.”