Defence officials are investigating after an employee from the UK Ministry of Defence lost several "sensitive" documents that were later found by a member of the public, a ministry spokesperson said.
The action follows a BBC report that the papers were found at a bus stop in Kent, southeast England.
"The Ministry of Defence was informed last week of an incident in which sensitive defence papers were recovered by a member of the public," the spokesperson told CNN in a statement on Sunday.
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"The department takes the security of information extremely seriously and an investigation has been launched. The employee concerned reported the loss at the time."
The BBC says it was passed the documents, which numbered almost 50 pages, by the member of the public when they realised the sensitive nature of the files.
The incident comes after a British warship - the HMS Defender - became embroiled in a confrontation with Russian forces off the coast of the disputed territory of Crimea on Wednesday.
Russia says one of its warplanes dropped bombs and a patrol boat fired warning shots to turn back the British destroyer it claims entered into its territorial waters in the Black Sea.
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The UK's Ministry of Defence denied Moscow's accusation, saying that the vessel was making a legal and innocent passage.
The BBC reports that the documents found at the bus stop discuss Russia's likely reactions to HMS Defender's passage on Wednesday.
Asked about the contents of the documents relating to HMS Defender, the Ministry of Defence spokesperson told CNN: "As the public would expect, the Ministry of Defence plans carefully. As a matter of routine, that includes analysing all the potential factors affecting operational decisions.
"HMS Defender conducted innocent passage through Ukrainian territorial waters in accordance with international law."
The lost documents also allegedly discussed a possible British military presence in Afghanistan following the end of the NATO mission in that country.
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CNN cannot independently verify the contents of the documents. The MoD did not comment on Afghanistan when contacted by CNN.
UK Secretary of State for Northern Ireland Brandon Lewis described the incident as a "serious breach" of security, speaking on the BBC's Andrew Marr show on Sunday.
"To an extent, processes were followed; it was reported by the individual as soon as they were aware they had left those documents and obviously somebody handed it into the BBC, but there will be a full investigation by the Ministry of Defence into how this happened," Lewis said.
"We've got to ensure when things like this happen, whether it's human error or otherwise, that it can't happen again."
