HMP The Mount at Bovingdon is struggling to employ enough officers before an influx of more "potentially volatile" inmates.
Personnel resources have already been described as "stretched to a limit" in a report in March.
But a new block, which will take the prisoner count from 770 to 1,008, is set to open in September and will include a "much higher number of shorter term (i.e. potentially more volatile) prisoners," according to a report by the Independent Monitoring Board.
Yet figures show that the prison is still failing to recruit enough officers, with the latest employment figures showing that the Molyneaux Avenue has 270 employees - 85 short of its target and the civil service is currently advertising for 50 prison officer jobs to make up for its shortfall.
The report states: "There have been permanent reductions in staffing levels through changes to operating profiles.
"Exacerbating the position is the very high level of staff vacancies at present.
"Ahead lies the opening of the new house block at which point the prisoner roll will increase by 33 per cent. The distances between various units will be considerably increased. The prisoner population will change to include a much higher number of shorter term (i.e. potentially more volatile) prisoners. The full complement of staff will rise from 313 to 351.5."
Dacorum Borough Council passed plans put forward by The Ministry of Justice to construct three new buildings including a new housing block, kitchen and sports changing block in February last year.
This is the latest in the series of expansions for the Molyneaux Avenue prison, which was originally built to house 480 youth offenders across five wings but has grown to accommodate 786 C category prisoners across eight wings.
The report by the IMB, which is completed by independent members of the public, also criticised the limited number of staff in the Care and Seperation Unit.
It said: "There are only three officers in the CSU from 7.45am to 6.30pm, reducing to one officer outside those hours. Essential training and maintaining the necessary standards has become an issue due to reduced staff levels.
"Board members have raised concerns with the governor on a number of occasions about the adequacy of CSU staff levels and the excessive pressure under which those staff work. The benchmark for B & C category prisons would seem to be focused on cost, not on need."
Figures obtained by the Watford Observer show that 450 staff members have left prisons across the East of England and nationally, nearly 5,500 staff have left the service. However, only 980 people have been hired.
Simon Letouze press officer for the prison service said: "The report highlights a successful year at HMP The Mount and rightly praises staff for their adaptability and commitment.
"Staffing levels at the prison are safe and have been restructured ahead of the opening of a new wing later this year, ensuring we continue to provide a safe and decent prison."