An evaluation by the Mental Welfare Commission indicated emergency measures taken to control the State Hospital budget earlier in the year had resulted in a reduced number of employees and resources being stretched.
The key role of the commission is to protect and promote the human rights of people with mental illness, learning disabilities, dementia and related conditions.
Their findings were published on Wednesday following visits to several hospitals across the country.
The report noted that high rates of sickness absence, staff retirals, financial pressures and high levels of enhanced patient observations were having a negative effect on patient care at Carstairs.
It also said the shortage of staff and a reduction in the availability of finances for equipment and resources had led to patients spending more time in their homes as the provision of activities became increasingly vulnerable to being cancelled.
Although patients at the hospital told the commission they found staff to be supportive and approachable, the evaluation document stated that staff morale had been negatively impacted.
The commission said managers at the hospital have made considerable efforts to address difficulties, but signalled their wish to be kept informed of the situation.
A spokeswoman for the hospital said: “The State Hospital management team recognises the critical role of its nursing staff in the delivery of safe and effective patient care and has taken significant steps to maintain its nursing workforce to its funded establishment.
“Workforce pressures have been experienced in recent years due to a combination of high levels of staff absence and clinical activity pressures requiring additional staffing. This can result in adjustments to care which are more restrictive but necessary to maintain staff and patient safety.
“The Board is proactively recruiting additional staff to a nursing pool which will provide further service resilience and has seen improvements in our staff attendance.”
Mental Health Minister Clare Haughey said: “Staffing at the State Hospital is one of the issues I raised with the chief executive during out discussions and he has sought to assure me that management are taking action.
“Recruitment continues and I have made clear my desire to meet with the management team to ensure they are working to understand staff concerns and take the necessary action.”
Scottish Liberal Democrat health spokesperson Alex Cole-Hamilton MSP said: “This report makes clear that dramatic cuts to staffing and resources are not the way to go.
“The Carstairs facility is constantly dealing with complex patient needs. In such circumstances hardworking staff cannot do their jobs if they are short handed.
“Patients and staff are being badly let down by this SNP Government’s handling of this facility.”