Prison officers are to be balloted on industrial action over pay, as union leaders warned staff of being on a "slippy slope back to the dark days of the 80s".

The Prison Officers Association Scotland (POA Scotland) said its members are frustrated over the "lack of progress around their inadequate levels of pay" as they agreed to the ballot at a conference in Perth on Friday.

The concerns come after union bosses claimed officers are having to deal with growing levels of violence, overcrowding and increasingly volatile environments.

POA Scotland has called for the Scottish Government to respond quickly.

Assistant general secretary Andy Hogg said: "This decision reflects the anger and frustration of our members over the lack of progress around their inadequate levels of pay.

"In 2014 it was accepted by the then justice secretary that a new pay structure should be introduced to recognise and reward Scottish prison officers appropriately for the challenging work they do. Five years on we are no further forward.

"This is coming at a time when our members are subject to increasing levels of violence, excessive overcrowding and an environment that is becoming ever more volatile through staff exposure to psychoactive substances and the violent behaviour of prisoners under their influence.

"We are on the slippy slope back to the dark days of the 80s and staff morale has never been lower. We have the makings of a perfect storm."

The union has not yet announced a date for the ballot.

Scottish national chairman Phil Fairlie said: "At a time when we should be sitting down discussing proper staffing levels for our prisons and a pay system that rewards and recognises the incredible work our members are doing every day, we are in fact being invited to discuss where to find in excess of £20m of savings due to the SPS budget settlement.

"Scottish Government need to listen to what we are telling them and respond quickly, before we find ourselves trying to manage a crisis in our prisons that is gathering momentum day by day."

The Scottish Prison Service (SPS) say they hold their Prison Officers in high regard and value the "commitment and dedication that they have shown for many years".

In a statement released on Friday they said: "Prisons in Scotland are under enormous pressure and it is to the great credit of all concerned with their operation that they continue to function safely and well.

"That the Prison Officers Association has taken the decision to ballot for industrial action over pay is a worrying development, particularly when the Service is under such strain.

"The SPS is already in discussions about pay and conditions with its Trade Union partners, including the POA(S).

"In conjunction with the Scottish Government, SPS will do everything it can to arrive at a fair and acceptable pay settlement.

"In doing so we hope that such positive engagement will ensure that the POA(S) will not in the future make the case to its members for industrial action."