The Ministry of Defence has admitted that secret information about the UK's nuclear powered submarines was made available on the internet by mistake.
A technical error meant blacked-out parts of an online MoD report could be read by pasting into another document.
Details were reported to include expert opinion on how well the fleet could cope with a catastrophic accident.
The MoD said a secure version had now been published and it was working to stop such an incident happening again.
Information also included measures used by the US Navy to protect its nuclear submarines, the Daily Star Sunday reported.
The document was an assessment by the head of the Defence Nuclear Safety Regulator, Commodore Andrew McFarlane, of the options for reactors for future submarines to replace the Trident fleet.
Some parts of the declassified dossier, published on the Parliament website, were redacted to protect sensitive information.
In the sections that were supposed to be released, it was concluded that existing reactors were "potentially vulnerable" to fatal accidents.
This caused ministers to suggest that different types would be used in future.
Conservative MP Patrick Mercer, a former Army officer, said the leak was "potentially catastrophic", with the details "hugely interesting" to the UK's enemies.
A spokesman for the MoD, which was told about the blunder by the Daily Star Sunday, 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."