Classified Ministry of Defence documents have reportedly been found at a bus stop in Kent.
Almost 50 pages, which contain details about HMS Defender and the British military, were found in a soggy pile behind the bus stop early on Tuesday morning, according to the BBC.
The person who found them, who wished to remain anonymous, reportedly contacted the broadcaster after realising the documents were sensitive.
The BBC reported that the papers included Russia’s possible reaction to HMS Defender passing through Ukrainian waters near Crimea on Wednesday.
They also reportedly contain plans for a possible UK military presence in Afghanistan after the end of the current NATO operation.
The Ministry of Defence (MoD) spokesperson said: "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."
They added: "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 department takes the security of information extremely seriously and an investigation has been launched.
"The employee concerned reported the loss at the time. It would be inappropriate to comment further."
Most of the papers are allegedly marked "official sensitive" - a relatively low level of classification.
But the BBC reports that one document addressed to the defence secretary Ben Wallace’s private secretary was labelled “Secret UK Eyes Only”.
It apparently outlines recommendations for the UK's military presence in Afghanistan, following the end of Nato’s Operation Resolute Support.
Watch: The moment a UK warship meets Russian forces
The document allegedly discusses the US requesting British assistance in several areas and whether special forces will remain in the country once the US troops’ withdrawal is complete.
It reportedy states: "Any UK footprint in Afghanistan that persists... is assessed to be vulnerable to targeting by a complex network of actors," but also "the option to withdraw completely remains."
The documents also reportedly show that the HMS Defender mission was conducted with the expectation that Russia might respond aggressively.
On Wednesday, more than 20 Russian aircraft and two coastguard ships followed the ship as it passed near Crimea's coast.
Russia’s defence ministry said a patrol ship fired warning shots and a jet dropped bombs in the destroyer's path but the UK government rejected this account.
The documents allegedly show the mission was the subject of high-level discussions as late as Monday night and three potential Russian responses were predicted, including from "safe and professional" to "neither safe nor professional".
A different route away from contested waters was reportedly considered.
But the documents said while this would have avoided confrontation, it ran the risk of Russia thinking this is evidence of "the UK being scared/running away" and accepting the country’s occupation of Crimean waters.
Watch: HMS Defender: Boris Johnson insists warship was sailing legally as Moscow says 'no options can be ruled out'