A new prison in East Yorkshire was "unready, understaffed and unsafe upon opening", according to an inspection report.

The Independent Monitoring Board (IMB) for HMP Millsike said a lack of staff and workers' inexperience meant "its opening was chaotic, with basic processes, like drawing keys, locking doors and counting prisoners, not yet established".

The Category C jail, run by Mitie Care & Custody and designed to house 1,500 prisoners, opened at a cost of about £400m last year.

In response to the report, Mitie said it valued IMB oversight but "we do not believe its description of the prison at the time it opened [is] a fair or accurate reflection of the conditions at HMP Millsike".

The IMB report said the remote location of the prison made hiring workers difficult, and staff "were in the main inexperienced, demonstrating uncertainty about the regime and processes, which impacted the safety, security and fair treatment of prisoners".

It also highlighted "design faults" in the prison including "blind spots for officers on landings, no gym, no air conditioning in workshops or houseblocks, and inadequate reception space".

However, the report said that "many of these problems have been tackled" following the introduction in February of an interim director.

HM Prisons and Probation Service (HMPPS) was criticised for appearing "to accept poor running of the prison on the grounds that such places take several years to settle".

The report said: "The board is disappointed that HMPPS considers inhumane treatment of prisoners to be an acceptable cost of that."

IMB Millsike chairwoman Rebecca Major said: "We've started to see improvements, but underlying difficulties remain."

Major said the intake of prisoners should have been paused until sufficient staff were recruited.

She added: "Unfortunately, the impact is already being seen on prisoners, with regime curtailments a daily occurrence.

"Without careful co-operation between Mitie and HMPPS, HMP Millsike will revert to being the unsafe, unfair and unhappy community we found last year."

In response, Mitie said it was working closely with HMPPS and the IMB.

"Since the prison opened, the overriding objective of our colleagues, working in challenging circumstances, has been to ensure that those within our care are treated with decency and respect, whilst creating a safe and secure environment that supports rehabilitation," a spokesperson said.

"We welcome the recognition of the improvements made since opening. The report highlights progress in operational stability, safety, healthcare provision, reception processes and complaint handling, as well as examples of 'excellent work' by our prison officers."

The BBC has contacted HMPPS for comment.