Warning letter submitted about Bedford Prison conditions
An official warning about conditions at Bedford Prison has been submitted to the justice secretary, the BBC understands.
It is the fourth jail to be subject to the "urgent notification" process after Nottingham, Exeter and Birmingham.
In May the publicly-run prison was placed in "special measures" by the government because of safety concerns.
Chief inspector of prisons Peter Clarke is understood to have sent a letter to Justice Secretary David Gauke.
The Ministry of Justice has been asked to comment.
The letter triggers a process which means Mr Gauke will have to publish a response and plan of action for the prison within 28 days, as well as a longer-term plan for sustained improvement.
The letter, detailing conditions inspectors found at the jail, is expected to be published on Thursday.
The Category B prison holds more than 500 male prisoners, although capacity is thought to be about 320.
A report by the Independent Monitoring Board also revealed the government had been warned of "an alarming decline in safety" at Bedford two months before the 2016 disturbance.