Hundreds of prisoners serving less than four years will be let out of jail over the summer after Scotland's prison population reached one of the highest levels ever recorded.

In a statement to the Scottish Parliament on Thursday, Justice Secretary Angela Constance told MSPs she would be seeking approval to use emergency early release powers.

The minister said there would be safeguards in place to make sure no one serving a sentence for sexual offences or domestic abuse would be allowed out.

Governors would also be given a veto to exclude anyone they believe "poses a risk to an individual or indeed to a group of individuals."

READ MORE: Six prisons at 'red' status as population hits near record high

The minister said there had been a sharp increase in the number of prisoners, jumping from 7,948 on 18 March to 8,348 on Thursday morning.

She said the cause of the 13% hike was "unclear".

Ms Constance said: "The 8,348 individuals in custody this morning represent one of the highest prison populations ever recorded in Scotland.

"There is now a critical risk to the continued safe and effective operation of the estate, with multiple prisons essentially full.

"The Scottish Prison Service’s ability to deliver rehabilitative regimes has been severely curtailed; visits to prisoners are becoming difficult to maintain, and there are increasing challenges to the effective delivery of NHS services."

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Ms Constance said the government had been taking several measures in recent months, including home detention curfew, and the introduction of electronically monitored bail. 

She said ministers were also now considering "whether compassionate release could be used more broadly in appropriate cases."

However, despite these steps, she said it was "increasingly clear that further action is required."

Ms Constance told MSPs: "The measures I’ve described will simply not have as large an impact as is necessary to avert a crisis.

"And be in no doubt, that is what we are facing.

"If our prisons are to remain functional and able to house the most dangerous offenders, we have no choice but to take urgent action to reduce pressure on the estate."

The Bail and Release from Custody (Scotland) Act - passed by MSPs last year and which takes effect on May 26 - gives Ministers the power to release prisoners in emergency situations. 

Ms Constance said she hoped the provision would never be needed but the threshold for taking emergency action had now been reached. 

To release prisoners early, ministers must show that it is necessary to ensure the “security and good order” of prisons or the “health, safety or welfare of prisoners, or those working in any such prison”.

Documents published by the Scottish Government said around 300-500 prisoners could be released under these measures. 

Only those serving sentences of less than four years and with 180 days or less left to serve can be considered for emergency release.

There would be a phased approach to the release, with all those eligible being let out over the next six months.

"We will, of course, be engaging with victims organisations, local authorities and other key partners in preparing for any release, and initial meetings are underway," Ms Constance said.

"This is not a decision I take lightly and I appreciate the concerns it will raise. But we must ensure the safety and wellbeing of SPS staff and those in their care, and that our prisons continue to function effectively to accommodate those who pose the greatest risk of harm."

Ms Constance said that the ending of automatic early release, under which all prisoners serving more than four years are freed after serving two thirds of their sentence, would be reviewed.

READ MORE: Prisoners may have to be freed early from packed jails, chief warns

Overcrowding in prisons south of the border has already led to the UK Government taking emergency measures to release some criminals early.

They have also triggered “Operation Early Dawn” which sees the start of some trials delayed some suspects released on bail rather than being sent to prison.

Scottish Conservative deputy justice spokesperson Sharon Dowey said: “Every prisoner is behind bars for good reason following a robust and independent judiciary process.

“Victims will not have been reassured by Angela Constance’s responses in relation to the risk posed to the public when these offenders are ultimately released back into our communities."

Phil Fairlie, Assistant Secretary of trade union, the Prison Officers Association said he was pleased the minister had recognised that prisons were "in a state of crisis."

"We have been making this case for some time. I am pleased to hear her refer to it in those terms and for her recognition of the continued risk it places on our members if it is not tackled as a matter of urgency. "

A Scottish Prison Service spokesperson said: “Our population, which was already very high and extremely complex, has increased rapidly in the past few weeks, in a way which was not anticipated.

“This is a critical challenge putting significant pressure on our staff, our establishments, those in our care, and our partners.

“The Cabinet Secretary's statement set out her legislative priorities and how she intends to engage with MSPs across the parliament to reach agreement on reducing the prison population.

“We stand ready to work with the Scottish Government, and our partners, to act upon the decisions reached, as we continue our focus on supporting our staff, those in our care, their rehabilitation, and Scotland’s communities.”

Kate Wallace, Chief Executive of Victim Support Scotland said they were "extremely concerned" about the plans.

"The motivation for this is the current highest ever prison populations, and the crisis that these figures bring.

"In essence though, the experience of the last time this was done in Scotland shows that we are only transferring this crisis to reoffending, drug deaths and further pressure on support services.

"Victim Support Scotland's primary concern is victim safety. We anticipate that many people whose perpetrator is currently in prison, will be concerned that they will be released and negatively impact on their day-to-day lives and safety."