Usk and Prescoed were named as high-performing prisons in 2006
Standards at two prisons remain good, despite dips in performance, a report has found.
A surprise inspection in March said both Usk and Prescoed in Monmouthshire were safe and clean but found staff were not "sufficiently trained".
Chief Inspector of Prisons Anne Owers said that while both prisons remained good, the inspection "revealed a degree of complacency".
Both are managed together along with Prescoed Young Offenders' Institution.
Usk is a closed jail for sex offenders, while Prescoed is an open prison which helps prisoners resettle in the community.
The inspection said Prescoed carried out its resettlement work "reasonably well", but found "impressive" offender behaviour work at Usk had been disrupted by a lack of psychologists.
Education and training opportunities at both sites were praised by inspectors.
Relationships between staff and prisoners were "relaxed" and race equality work was found to be good.
Ms Owers said: "Overall, Usk and Prescoed remained good prisons, focusing on their different and important roles.
"However, this inspection revealed a degree of complacency and some drift, which needs swiftly to be remedied if the prisons are to regain and retain their high-performing status."
Phil Wheatley, the Director General of the National Offender Management Service, said he agreed that Usk and Prescoed remained good prisons which were focused on different tasks.
"The underpinning quality of staff/prisoner relationships is also very good and the chief inspector confirms this.
"The report will act as a spur to the governor and staff to continue to build on a high quality performance.
"Already the clustering of forensic psychological resources across two Welsh prisons and an additional lead role for resettlement at Usk are bringing improvements to performance," he said.
The unannounced inspection took place between 3 and 5 March this year.
In 2006, the prisons joined only six others in the UK to be given high-performing prison status.