The Independent Monitoring Board (IMB) said Bristol Prison had fewer prison officers on the landings than last year and the situation had become "serious".
It also found the prison was infested with cockroaches.
But prison governor Andrea Albutt said new procedures had been introduced and in her view it was a safe environment.
The Category B prison, in Horfield, holds more than 600 male adults serving short sentences or on remand.
The latest report, which followed a critical inspection in 2013, acknowledged that despite "major budgetary cuts" there had been "improvements" in the induction process for new prisoners and the number of inmates in education or work.
But it said staffing levels on residential wings were at times "insufficient" to ensure a safe environment for prisoners and staff and wings were being "placed in lockdown" as a result.
"The new ways of working, the prison is undergoing, means there are less prison officers on the landings then there were 18 months or a year ago and this is having a direct knock on effect," said Dr Michael Flannery, chairman of the IMB for Bristol Prison.
"The prison is at bursting point and it's a serious situation and we are very concerned."
But Ms Albutt, a prison governor, said she did not "necessarily accept" the IMB findings.
"What makes a prison safe is staff-prisoner relationships and in Bristol we have excellent staff-prisoner relationships," she said.
"Last month Her Majesty's Inspectorate of Prisons inspected us and consistently saw excellent staff-prisoner interaction and a prisoner survey that was carried out at the same time said that prisoners felt safe."
The IMB report also highlighted issues over frequent shortages of prison clothing and bedding, cockroach infestations and the cleanliness of some communal showers and toilets.
A jail where low staffing levels had made prisoners and staff feel unsafe is now at "bursting point" with even fewer officers, a new report has found.