A tabloid newspaper, the Daily Star, alerted the department to the fact that certain sections of their website, which had officially been redacted, or blackened-out, could be read by simply cutting-and- pasting the text into a new document.
The offending paragraphs - which have now been properly excised from the web - reportedly included expert opinion on the fleet's ability to withstand a catastrophic accident.
Other details included information about how the US Navy protects its nuclear submarines.
An MOD spokesman said: "The MOD is grateful to the journalist for bringing this matter to our attention.
"As soon as we were told about this, we took steps to ensure the document was removed from the public domain and replaced by a properly redacted version. We take nuclear security very seriously and we are doing everything possible to prevent a recurrence of this."Authors: Britta Guerke, Matt Tempest