LONDON (AFP) - British police were trying to quell a prison riot involving up to 200 inmates, which broke out just days after a warders' association warned of a "bloodbath" in the UK's detention facilities.

Police, ambulance and fire services were called to the prison in Bedford, central England, shortly before 5:00pm Sunday after the disturbance broke out.

The Prison Officers Association's (POA) general secretary, Steve Gillan, estimated up to 200 prisoners were involved.

"Prison officers on the landings have withdrawn to a safe place, so prisoners are out in large numbers.

"We don't know if it will turn into a full-scale riot as these things can turn very quickly," he said.

A police spokeswoman said a number of officers were working with staff to resolve the incident at Bedford prison, which currently holds around 500 inmates.

Following the outbreak of unrest a Prison Service spokesman warned there would be consequences for those involved: "We are absolutely clear that prisoners who behave in this way will be punished and could spend significantly longer behind bars."

The incident comes just days after the head of the POA, Mike Rolfe, warned of the deteriorating situation in Britain's prisons.

"It's a bloodbath in prisons at this minute in time. Staff are absolutely on their knees, lost all morale, all motivation," he told BBC Radio Four.

"Low staffing numbers, people leaving the job in droves, it's a real bad mix, and it's dangerous for everyone, staff and prisoners alike."

Last month a 21-year-old inmate was stabbed to death at London's Pentonville prison.

On October 29 a national response unit was brought in to control prisoners at East Sussex prison, south-east England, in a standoff which last six hours.

On Friday the justice ministry announced the creation of 2,500 new jobs as part of its broader prison reform.

The government also promised annual investment of more than £100 million and greater efforts to tackle drugs in prisons.

The worsening situation at Bedford prison was documented in a September report by watchdog HM Inspectorate of Prisons, which found the number of inmates saying it was easy or very easy to get drugs had almost doubled since February 2014.

The report also documented shortages of clothing, cramped conditions, and weak arrangements for managing violent and bullying behaviour.