Emergency legislation that will see hundreds of prisoners released from Scotland’s jails early is only one of the steps needed if the “real issue” of prison overcrowding is to be tackled in the longer term, MSPs have been told.

The Prison Officers’ Association (POA) said it supported a Scottish Government Bill which will bring forward the point at which offenders serving shorter sentences become eligible for automatic early release.

POA assistant general secretary Phil Fairlie said the Prisoners (Early Release) (Scotland) Bill was a “step in the right direction in helping to reduce prisoner numbers”.

But he added it was just one “of a series of steps that will be required if the very concerning and real issue of prison overcrowding is to be tackled longer term”.

Mr Fairlie stressed there is a need for a “much wider and fundamental debate in Scotland about what it is we want from our prisons” as well as a review of sentencing policy in Scotland.

In a letter to MSPs, he added: “In the meantime, however, our members are tired, they are stressed, they are being assaulted in increasing numbers, they are having to intervene into prisoner assaults at an increasing level, while dealing with more and more issues of complex mental health deteriorations and self-harm episodes amongst the prisoner groups, which in turn is all having significant impacts on the mental health of our members too.

“We are grateful that the extent of the problem appears to be getting recognised and that this step is one at least that has the ability to make a small, but very much needed and welcomed contribution to helping to ease the enormous strain, while the longer term more permanent solutions are sought.”

His comments came as MSPs are expected the pass the legislation – which is being accelerated through Holyrood as ministers seek to reduce the high number of inmates inside Scotland’s jails

Under the plans, between 260 and 390 prisoners would be released by early February in three tranches over six weeks.

The Bill changes the automatic release point for sentences of four years or less from 50% to 40% of their term – although those convicted of sexual or domestic violence would be excluded.

Last week, support from SNP and Green MSPs meant the Bill passed its first stage – with the final two stages of the legislative process due to be completed on Tuesday, with Holyrood sitting late as a result.

But Labour, the Tories and the Liberal Democrats voted against the legislation.

It comes as prisoner numbers in Scotland have increased again after 477 offenders were released early this summer, with ministers using emergency measures in a bid to try to tackle overcrowding.

Speaking in Holyrood last week, Justice Secretary Angel Constance said: “The changes made in this Bill are absolutely critical to relieve pressure on the prison estate and address the current crisis.”

But Scottish Liberal Democrat justice spokesperson Liam McArthur branded measures in the Bill a “short-term fix”, as he argued that the accelerated timetable for the legislation could hinder parliamentary scrutiny.

Mr McArthur added he had “grave concerns” about parts of the Bill which he said “give Scottish ministers far-reaching powers to release any prisoner at any time with limited parliamentary scrutiny”.

The Liberal Democrat added: “This is a step too far, comes with significant risks and is not something Scottish Liberal Democrats can support.”

The Association of Scottish Police Superintendents (Asps) said it had “grave concerns” about the early release of prisoners.

Asps president Rob Hay noted than when 477 prisoners had been released early in June and July of this year, more than 10% ended up back in prison “within weeks” because of offending, with some “having committed violent offences”.

He said: “The public are being put at risk. Every crime committed and every victim who suffered at the hands of those criminals represent an avoidable consequence of this misguided Scottish Government policy.

“To release prisoners early, knowing further offending is likely to occur, piles pressure on a police service already stretched to breaking under unsustainable demand.

“The only comfort I can offer the public is that where police officers encounter released prisoners who remain involved in crime, we’ll do all we can to ensure they end up back in custody, where they belong.”