It follows a routine visit to the dockyard's nuclear submarine refit complex by the health and safety executive's Nuclear Installations Inspectorate (NII).
Inspectors were concerned with the procedures they found in place for inspecting and maintaining plant at the Plymouth yard.
The BBC has discovered that last month inspectors took the step of issuing a safety improvement notice against DML.
Trafalgar class
Devonport is the largest naval base in Western Europe, covering over 650 acres, with 15 dry docks, four miles of waterfront, 25 tidal berths and five basins.
It is home to seven Trafalgar class nuclear submarines.
This is the first time the yard has had a NII warning about nuclear safety for three years.
But it follows concerns earlier this year over the brand new Trident refit docks, which the NII initially refused to fully licence.
The safety notice gives DML a year to comply with the upgrade, indicating that there is no immediate risk.
The company said regulators were now putting increased emphasis on formal safety assessments.
DML will now have to accelerate its programme of assessments at the nuclear installation.
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