Two rows of individual "bleak cages" are being used as a grim exercise yard for segregated inmates at one of the UK's top security jails, the chief inspector of prisons has disclosed.

Nick Hardwick says the cages at Long Lartin prison in Worcestershire, which holds some of the country's most dangerous prisoners, are "not fit for purpose" and their use is unacceptable.

He also sharply criticises the continued Victorian practice of "slopping out" at Long Lartin, despite its supposed abolition in jails in England and Wales in 1996.

The chief inspector says the night sanitation system at the jail involves prisoners pressing a button to join a queue so they can leave their cell one at a time to use a toilet.

But he says the wait between 8pm and 1am can be as long as two hours and prisoners usually resort to using a bucket in their cell, which they empty in the morning.

"Whatever the official title, this was slopping out," he says in his report on the prison published on Tuesday.

Michael Spurr, the chief executive of the national offender management service, who heads the prison service, disputed the finding. "The system is designed so that prisoners have access to appropriate sanitation at night and should not have to routinely 'slop out'," he said.

Hardwick says his inspection of conditions last August inside Long Lartin , which in the past has contained Abu Qatada and Jeremy Bamber, showed the jail was successfully holding some of the most challenging prisoners in the system, but there were some significant concerns.

He says most of the 611 prisoners, more than half of whom are serving life sentences, felt safe and there were relatively few violent incidents although those that did occur were often serious.

But he says many prisoners on the vulnerable prisoner wings said they did not feel safe and prisoners on suicide watch were often held in the segregation unit, which had a limited regime, staff who often appeared uninterested, and a grim "exercise yard".

The prison inspectors found that conditions inside the very large segregation unit were reasonable but some cells, particularly the two "constant watch" cells, were dirty.

"The gated exercise yards were little more than bleak cages and not fit for purpose," reports the chief inspector.

Hardwick also says Long Lartin needs much more help from the national prison service to work successfully with the 25% of its prisoners who are Muslim.

He discloses that those held in the detainee unit – mostly those facing deportation on national security grounds – had been denied permission to celebrate Eid al-Fitr, the holiday that marks the end of Ramadan, with other Muslim prisoners.

Hardwick said Long Lartin provided a reasonably safe environment for most prisoners, most of the time.

"Some aspects of the prison, such as healthcare, were very good," he says.

"However, other aspects – the segregation unit and slopping out – were unacceptably poor. The contrasts in the perceptions of vulnerable and mainstream prisoners were striking.

"The fears of vulnerable prisoners for their safety need to be taken seriously, understood and addressed, as do mainstream prisoners' concerns about they way they are treated."

Spurr responded: "There is more to do and the governor and his staff will continue to work on those areas highlighted for improvement in the report while balancing the need for a constructive regime for a very challenging population with the core priority of security."ends