The UK's newest prison has had a "chaotic atmosphere" in its first 18 months, former workers have told a BBC investigation.
HMP Millsike, in East Yorkshire, was heralded by the government for setting "the standard for the future" when it opened in March 2025 with 1,500 places.
But former workers have described under-staffing, inadequate training and seeing specialist officers called in to deal with serious incidents on numerous occasions.
Mitie, the company that runs the category C prison, said safety was its "highest priority" and it was "operating in line with plans agreed with the Ministry of Justice".
Millsike was built with a focus on rehabilitation and education and cost about £400m.
It was the first of four new jails due to be opened under a government pledge to create 14,000 places by 2031.
About 1,100 inmates are held there at present, according to the Ministry of Justice (MoJ).
But there have been a number of issues since the prison opened, including:
Reports of a disturbance in November 2025, with a former worker stating a prisoner "lost it" and an elite prison squad had to be called in.
A sewage pumping station breaking down in the same month, amid reports that inmates had flushed clothes down toilets, which Mitie denied.
The conviction of a female prison officer, who was given an eight-month, suspended jail term in June 2026 after admitting to a relationship with an inmate.
The BBC spoke to seven former employees, including a prison officer and staff members.
All but one wanted to remain anonymous, but described a "chaotic atmosphere" inside Millsike.
Six spoke of staffing shortages, and four said they felt unsafe working there.
Several expressed concerns about the experience of other staff, claiming some had backgrounds working in supermarkets and fast-food chains.
The BBC understands all operational staff complete more than eight weeks of training, which includes shadowing roles in other prisons.
The MoJ said Millsike would employ about 600 people, external, but it is unclear at present how many roles are filled as private prisons, unlike public ones, do not have to publish this data.
Mitie declined to tell us the number of current vacancies.
During a public meeting in June, the BBC understands interim prison director Paul Newton told local people recruitment was difficult and 90% of employees had never worked in the prison sector.
Mitie said "recruitment and training aligned to the prison's phased growth" and that it recruits from other backgrounds, such as the military and other areas of the criminal justice network.
Three former staff members, working in non-prison officer roles, described how the shortage of officers meant they had done tasks not in their job descriptions.
This included "bed-watching" vulnerable prisoners at risk of harming themselves, despite not being trained to do so.
Steve Richardson, who worked in maintenance for 10 months, was among them.
He described having "faeces and urine" thrown at him by inmates and claimed staff had to "mix in" and help one another.
Richardson claimed he was not given control and restraint training, known as DoJo training, which helps prison staff deal with violent situations.
Mitie said everyone who needed DoJo training had received it.
Richardson recalled a night in November when an inmate "absolutely lost it and he started throwing pool balls and cues around", smashing six windows.
He said the inmate tried to climb a fence, but an elite prison squad from the Operational Response and Resilience Unit (ORRU) "stepped in and took him down".
The ORRU is made up of specialist teams, including a dog unit, and is called in to deal with the most serious high-risk incidents.
Units were deployed to Millsike seven times in nine months in 2025, according to data provided by the MoJ under a Freedom of Information request.
Of the other privately run category C prisons in England we received data for, units were called eight times to Oakwood and three times to both Fosse Way and Northumberland.
Mitie said call-outs were in line with expectations.
In a separate incident on 9 July, people living near Millsike described seeing a bomb squad vehicle entering the site, followed later by a loud bang.
Messages sent by inmates that evening – seen by the BBC – tell of alarms going off and the sound of an explosion.
Humberside Police and Humberside Fire and Rescue Service confirmed they attended along with an explosive ordnance disposal team.
Mitie said no viable device was found and emergency services attended in line with standard procedures.
In January this year, the Independent Monitoring Board (IMB) for the prison raised concerns about safety and security.
A report said its members visited in pairs due to "feeling unsafe on the wings" and had noted doors and gates left open.
The following month, the prison's director, Samantha Pariser, was replaced, with Mitie stating she had moved to a "new strategic role".
The Prison Reform Trust said it had received some calls from prisoners about their safety.
Chief executive Pia Sinha said this was "not unusual", but "some of the calls have been worrying and concerning".
The trust raised safeguarding concerns with the prison, which had been "responsive".
Sinha said some of the challenges Millsike was facing had been seen in other new prisons.
"We do need to cut them a bit of slack because it takes time," she added.
Mitie said safety was the "highest priority" and it maintained "a safe, secure and supportive environment for prisoners, colleagues and visitors".
PeoplePlus, which delivers rehabilitation programmes, said: "Education and industry services have developed in step with the prison's population."
The MoJ said Millsike was "a vital part of our plans to build 14,000 new prison places".
"A robust procurement process" had been conducted before the contract was awarded and it would "act when standards fall short".
The BBC has been told an annual report by the IMB, due to be published within the next month, will address safety concerns.
