Illegal brewing of alcohol and drug smuggling have become "major problems" at Dartmoor Prison, according to a new report.
The Independent Monitoring Board said substances had been thrown over the walls of the 200-year-old prison until grills were placed on the windows.
Its annual report also revealed some prisoners were being required to double-up in single cells, essential refurbishment work to the fabric of the building had been delayed, and an increasing proportion of vulnerable prisoners had put strain on prison resources.
The board also expressed disappointment that some of the issues raised in last year's report had not been significantly improved, including a shortage of work spaces and concerns about healthcare provision.
Although the report said quantities of drugs had been reduced by cell searches and drug testing, it admitted prisoners were still able to obtain illegal drugs and described the situation as an "ongoing battle".
"Some of this results from prisoners selling their own medication, having that medication stolen or being bullied, but drugs are also entering the prison by other routes," the report said.
It added: "The dog handler was successful on two occasions in intercepting a quantity of drugs and mobile phones that were being thrown over the wall."
Like the previous update, the latest report for the year ending October 31, 2012, identified "insufficient" work placements for Dartmoor prisoners. It also noted an "apparent reduction" in education classes and workshops. Other concerns about cell space, refurbishment and healthcare were highlighted in the latest report.
Sixteen cells designed for single occupancy were regularly used for two prisoners, which the report described as "not acceptable in terms of privacy, hygiene and decency". It also noted another 14 twin cells were "no bigger than single cells".
The board said healthcare provision was "inconsistent". It said the prison had suffered from a shortage of staff, medication issues, poor service out-of-hours, and a lack of locum doctors.
Its report said the lack of investment in maintaining the prison's fabric was a "serious concern, especially as it impacts on the decency agenda".
But the board praised the prison in several ways. It said the management team had provided "strong direction" leading to increased stabilisation and added that Dartmoor was a "safe prison with low rates of violence" which had a "welcoming atmosphere for families."
In the overall judgement of the prison, the report said there was "a lot of uncertainty about the future".
A Prison Service spokesman said: "The report by the Independent Monitoring Board into HMP Dartmoor will be fully considered by Ministers and we will respond in due course."
Dartmoor Prison, situated on the western edge of the Dartmoor National Park, is a category C prison with 659 adult male prisoners. Two of its six units are for vulnerable inmates.