The UK Government has started an investigation after top secret British defence documents were found at a bus stop.
Some of the Ministry of Defence documents revealed Russia's expected reaction to a UK ship passing through Ukranian waters off the coast of the annexed Crimea on Wednesday
The documents discussed details about HMS Defender and the British military and were found just hours before the incident took place.
The papers, which ran to almost 50 pages, are said to have been found in a wet pile behind a bus stop in Kent on Tuesday morning.
They cover a range of subjects - including the expected Russian reaction to the Royal Navy's stand-off with Russia in the Black Sea.
Another document in the abandoned pile relates to the UK's plans for a potential continued military presence in Afghanistan after Nato's withdrawal.
A Military of Defence spokesman told the BBC that a worker had reported the loss of secret defence papers.
They added: "It would be inappropriate to comment further."
And the BBC said the UK Government said it had launched an investigation into the incident.
The documents are said to reveal that the MoD thought Russia may react aggressively before HMS Defender sailed through waters off the coast of Crimea, which was illegally annexed by Russia in 2014.
The BBC believes the documents, which include emails and PowerPoint presentations, originated in the office of a senior official at the Ministry of Defence (MoD).
The documents relating to the Royal Navy's Type 45 destroyer, HMS Defender, show the MoD planned an "innocent passage through Ukrainian territorial waters".
The operation would see guns covered and the ship's helicopter stowed in its hangar, and was being conducted in the expectation that Russia might respond aggressively.
On Wednesday more than 20 Russian aircraft and two coastguard ships shadowed the warship as it sailed about 12 miles off the coast of Crimea.
Moscow's defence ministry said a patrol ship fired warning shots and a jet dropped bombs in the destroyer's path but the UK government denied any warning shots had been fired.
The documents reveal the mission, dubbed "Op Ditroite", was the subject of high-level discussions as late as Monday, with officials speculating about Russia's reaction if HMS Defender sailed close to Crimea.
And one presentation warned: "RFN (Russian navy) and VKS (Russian air force) interactions will become more frequent and assertive."
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