About 80 prisoners were involved in the unrest at Shotts Prison in Lanarkshire, which was finally brought under control at about 1500 GMT on Friday.

Inmates had earlier refused to give up their protest despite appeals from prison officials.

Two prison officers needed hospital treatment, one for a broken leg, when trouble flared in two maximum security halls on Thursday evening.

There are preliminary investigations ongoing as we speak and there will be prison service as well as police involvement A prison service spokesman

Police and prison bosses have joined forces in a bid to establish what sparked the disturbance.

A spokesman for the Scottish Prison Service (SPS) said preliminary inquiries had started.

However, he refused to be drawn on reports that the incident was a result of a crackdown on drugs in the jail.

He said: "Because drugs are a problem in all Scottish jails, these are actions that are ongoing.

Police involvement

"We have drug sniffer dogs in the prison service, drug detecting equipment, monitoring of phone calls and CCTV - so cracking down on drugs is what it's all about.

"To say that this was what caused the incident at Shotts is just speculation."

The spokesman added: "There are preliminary investigations ongoing as we speak and there will be prison service as well as police involvement.

"Early indication suggest there was some damage to the prison building, so that will be investigated too.

"We will interview staff and prisoners, but things are at an early stage just now. We're still gathering information and establishing what happened and why it happened."

Police were stood down on Friday afternoon

All prisoners at Shotts are serving four years or more, including many serving more than ten years to life for serious crimes such as murder.

The prison is also the base for Scotland's National Induction Centre, where all inmates sentenced to ten years or more serve their first 12 months.

The SPS said the prison, built in 1978, was currently running at just below capacity with about 540 inmates at the 550-560 space jail.

The prison, which is sited near the village of Shotts, has been affected by unrest in the past.

There was a stand-off in April last year between prisoners and authorities after an electrical storm cut off supplies to the facility.

On one occasion in the early 1990s a group of inmates wrecked their cells while on another a prison officer was held hostage.