An MSP has described conditions at HMP Barlinnie as "gravely concerning" after a visit by Holyrood's Justice Committee as part of its budget scrutiny process.
A replacement for the Victorian prison has been slated to be built by 2025, with the Scottish Government in talks to buy land in the north-east of Glasgow for the new building.
During the visit, MSPs were told there were 1,400 prisoners housed in the building, which has a design capacity of 987.
Committee members spoke to staff at the jail, who reported 90% of prisoners being forced to share cells, while just five cells are suitable for disabled inmates.
Committee convener Margaret Mitchell said: "The conditions in HMP Barlinnie are gravely concerning.
"While the committee pays tribute to the hard work of the staff, it is not an environment conducive to rehabilitating prisoners.
"The prison is not designed for modern-day needs and is unacceptably over-crowded, with prisoners sharing small, cramped cells and using dilapidated facilities."
She added: "The committee has taken a wide look at post-conviction spending and will make detailed recommendations in due course.
"However, it is clear the issues at Barlinnie in particular will have to be addressed as a matter of urgency."
A Scottish Prison Service (SPS) spokeswoman said: "The Scottish Prison Service is committed to modernising and improving our prison estate.
"We recognise the need to replace our older prisons and are working with Scottish Government to secure the necessary funding.
"Given the high number of people in custody and the age of the infrastructure, the operational priority for the SPS is a replacement for HMP Barlinnie and we are in the final stage of the purchase negotiation process to purchase a site for the new prison.
"We are currently considering what interim measures can be taken to improve conditions ahead of the replacement prison being constructed."
A Scottish Government spokesman said: "The Scottish Government and Scottish Prison Service (SPS) are committed to modernising and improving our prison estate.
"We take very seriously the pressures facing prisons and we have been working closely with the SPS to manage the rising population.
"We are clear that recent increases in the prison population are having an impact on our prisons and, in particular, on the conditions within HMP Barlinnie.
"The Scottish Government and SPS are looking at interim measures to address conditions within the prison ahead of its replacement."