Inmates at Bristol prison were admitted to hospital seven times over six-months after taking banned legal high Spice, it has been revealed.
A report by HM Inspectorate of Prisons found that a mandatory drug testing regime did not detect Spice, a synthetic drug used to imitate the effects of cannabis.
It is believed to have become one of the main drugs of choice inside the jail.
The Inspectorate praised Bristol Prison for making considerable progress but says significant concerns still remain.
They found the most serious issue identified at the prison was the level of violence, which was much higher than in similar prisons.
The Justice Secretary Chris Grayling said he regretted the decision to close the prison:
The Ministry of Justice said work to close HMP Dorchester will begin immediately and it is expected to shut close at the end of the year.
Everything will be done to avoid compulsory redundancies and our intention is for all staff to be redeployed to alternative roles in the prison estate or, if necessary, offered the opportunity to leave on voluntary terms.
Offenders housed at Dorchester will be moved to other suitable prison accommodation.
The Ministry of Justice has announced the closure of Dorchester Prison.
It is part of the Government's prison modernisation programme to replace old and outdated estates.
A planned new 2,000 place prison will be built in Wrexham.
A feasibility study has started on a second large prison to be constructed in the South East of England.
Seven jails in the South West could be turned into 'settlement' prisons, and would see the majority of offenders released from prisons in, or close to, the area in which they live.
Home Office Minister Damian Green has spoken to ITV News West Country:
Seven jails in the South West are included in proposals to become 'resettlement prisons'.
The plans, which are being revealed by the Justice Secretary Chris Grayling on Thursday, will see the majority of offenders released from prisons in, or close to, the area in which they live.
- Leyhill in Gloucestershire
- Channings Wood in Newton Abbot
- Guys Marsh in Dorset