The Prison Service has launched an investigation into a prison where riot police spent six hours getting prisoners under control.

Police and specialist guards were called to HMP Bedford after a riot reportedly broke out.

Up to 200 inmates are thought to have been involved in the incident, which was "successfully resolved" at around 11.30pm on Sunday night.

Riot police seen inside the prison during the incident. Credit: ITV News

Some guards retreated during the riot, the Prison Officers Association (POA) said, and national teams of riot-trained officers were deployed to the prison.

Emergency services were called at around 5pm, with ambulance and fire services on standby outside.

A spokesperson for the Prison Service said: "Specially trained staff are working with the emergency services to resolve an ongoing incident involving a number of prisoners at HMP Bedford.

"We are absolutely clear that prisoners who behave in this way will be punished and could spend significantly longer behind bars."

Up to 200 inmates are thought to have been involved in the riot. Credit: ITV News

The riot came after the head of the POA Mike Rolfe last week warned British jails have been engulfed by a "bloodbath".

Steve Gillan, POA general secretary, said: "The POA has been warning about this situation of violence in our prisons - it would appear it's coming to fruition.

"I just hope there's no prisoners or indeed prison officers are injured in the violence."

Specially trained police officers had to be brought in to deal with the situation. Credit: ITV News

HMP Bedford currently holds around 500 inmates and according to a HM Inspectorate of Prisons report in September, inmates said it was easier to get drugs than clothes or bedsheets.

A survey found the number of prisoners saying it was easy or very easy to get drugs had almost doubled since the last inspection of the jail in February 2014.

The number saying they had developed a drug problem while at the prison increased from 4% to 14%.

Some guards in the prison were reported to have retreated to a 'safe place'. Credit: ITV News

The HMP inspection in May also found that the physical condition of the prison was poor, with many inmates living in cramped conditions.

The report detailed damaged furniture, graffiti, shortages of clothing and dirty, unscreened showers.

The report also said: "Arrangements for managing violent and bullying behaviour and supporting victims were weak."