British Police have been called to a Bedford prison following reports of a riot.

Officers and ambulance crews were called to the prison in the town centre at around 5pm.

Steve Gillan, general secretary of the Prison Officers Association (POA) said there could be “anything up to 200 prisoners involved.”

“We don't know how many are out of the cells, some are still locked up,” he said.

“Prison officers on the landings have withdrawn to a safe place, so prisoners are out in large numbers.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

A Bedfordshire Police spokeswoman confirmed police are assisting the prison authorities following “reports of unrest.”

"We were called shortly before 5pm and have a number of officers at the scene working with prison staff to resolve the issue," she said.

We're currently working alongside partner agencies to assist HMP Bedford following reports of unrest within the prison earlier today

— Bedfordshire Police (@bedspolice) November 6, 2016

A POA spokesman said specially trained staff are working with the emergency services to resolve the situation.

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

Fire services are also on standby outside the prison.

We are on standby outside Bedford Prison, no indications of any fires inside

— Beds Fire and Rescue (@BedsFire) November 6, 2016