A pool cue was used to hit the prison officer on the head
A second prison officers was injured in a disturbance at Scotland's newest prison, it has emerged.
Emergency services were called to Addiewell Prison in West Lothian after up to 10 prisoners rioted on Monday.
One of the officers was hit by a pool cue in the incident. It is not known how the second officer was hurt.
Neither of the officers is thought to have sustained serious injuries. The incident was brought under control a short time later.
It is understood the disturbance was sparked after an inmate was told his methadone was to be reduced.
Steve Farrell, of the Prison Service Union, told BBC Scotland that the incident had started at about 1915 GMT on Monday in the prison's Lomond 'B' Hall as the result of a discussion between a prisoner and officer over medication.
The inmate began abusing the officer before striking him with a cue, Mr Farrell said.
He added: "The prison officer did the correct thing, pressed his alarm and vacated the hall. At that time the rest of the prison was operating perfectly normal.
"In one wing the prisoners were being disruptive and the staff - at around quarter past nine - went into the hall, as they would do in a normal intervention, and took the hall back safely without any injuries to prisoners or prison officers."
Mr Farrell said the incident was "not unusual" in any prison, whether public or private, especially in a relatively new prison which is more likely to be more inexperienced staff.
"Addiewell officers again last night showed how capable they are in responding to such incidents because I am told the intervention was first class, as good as anywhere in the prison service in Scotland," he added.
Mr Farrell said he believed privatisation was the "correct way forward" for the prison service, but it had to be supported by external authorities, including the Scottish Prison Service.
A spokesman for private company Kalyx, which manages HMP Addiewell for the Scottish Prison Service, said: "We can confirm that a contained incident, involving a small number of prisoners, took place in one of the wings at HMP Addiewell and was brought under control last night.
"There has been minor damage caused, mainly as a result of burst pipes.
"Two prison officers were injured during the incident and were treated at hospital but have now been discharged."
Two ambulance special operations response teams stood by outside the jail from 2030 GMT until 0100 GMT following reports of an ongoing riot in the prison.
Lothian and Borders Fire and Rescue Service also waited outside until shortly after midnight after a fire alarm inside the jail was set off at about 1930 GMT.
A former chief inspector of prisons in Scotland told BBC Radio Scotland's Good Morning Scotland programme that he believed some prisoners were taking advantage of lower staffing levels in privately-run jails.
Clive Fairweather, who has previously inspected the private prison at Kilmarnock, said they had to make a profit, which can put pressure on staff numbers and training.
He added: "The difference between a privately-run prison and the rest of the prison estate is that first and foremost it's innovative and really has cutting-edge systems, but it comes with a bit of a problem which the governor always has to overcome, which is it's got to make a profit.
"To make a profit the only place you can cut corners is on staffing.
"Therefore you have the minimum number of staff, you have the minimum amount of training and it's certainly my experience with Kilmarnock that violence and the like was a problem until eventually staffing levels got to a slightly better stage."
Mr Fairweather said prisoners in jails such as Addiewell and Kilmarnock have "never had it so good".
But he added: "They didn't want to go anywhere else, but they are taking advantage of the fact there aren't the same staffing levels as there are in other major prisons.
"Indeed, were there to be major riots in somewhere like Addiewell or Kilmarnock, I'm pretty certain the riot shields and those to deal with it would actually have to come from the rest of the SPS."
HMP Addiewell opened in December 2008 and holds up to 700 prisoners.
A prison officer needed hospital treatment after a disturbance involving about 20 inmates in October last year.
In February five men were placed in a segregation unit after a three-hour disturbance, during which one cell was damaged by fire.