Increased levels of violence caused by psychoactive substances had seen UK Prison, HMP Wealstun inmates and staff needing hospital treatment, the Independent Monitoring Board (IMB) revealed.
Its report said staff at the jail near Wetherby were dealing with issues involving drugs up to 10 times a day.
The Prison Service said it had increased the number of drug searches.
The IMB noted however, the prison was “well-managed” with inmates treated “fairly”.
Inspectors visited the category C prison multiple times between June 2016 and May 2017.
“Bogus” correspondence purporting to be from legal representatives was a “significant problem”, the report said.
Prison rules stipulate staff are only able to open such letters if they had suspicions they contained banned items.
It continued: “Where there is suspicion that incoming letters have been soaked in drugs, they are photocopied. This has resulted in an increase in complaints from prisoners, as bogus legal mail is a significant problem and legal mail should not be opened.”
A “considerable” increase in assaults at the prison was linked to increased drug use, coupled with “the frustration prisoners feel when regimes are curtailed due to lack of staff”.
A Prison Service statement said: “We take a zero tolerance approach to drugs in our prisons, and are clear that anyone found with contraband will be subject to disciplinary action and police investigations,”
“While there remains progress to be made, HMP Wealstun has already taken action to improve security and remains proactive in its tackling of drugs and other contraband.
“The prison has also increased the number of intelligence-led drug searches and has secured funding for high-definition CCTV across the site.”
Built on a former munitions site, the prison has a capacity for more than 800 inmates.