Classified Ministry of Defence documents containing details about HMS Defender and the British military have been found at a bus stop in Kent.
One set of documents discusses the likely Russian reaction to the ship’s passage through Ukrainian waters off the Crimea coast on Wednesday.
Another details plans for a possible UK military presence in Afghanistan after the US-led Nato operation there ends.
The government said an investigation had been launched.
The Ministry of Defence (MoD) said it is investigating “an incident in which sensitive defence papers were recovered by a member of the public”. The MoD employee concerned reported the loss at the time, it said.
The documents, almost 50 pages in all, were found in a soggy heap behind a bus stop in Kent early on Tuesday morning.
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A member of the public, who wishes to remain anonymous, contacted the BBC when he realised the sensitive nature of the contents.
The BBC believes the documents, which include emails and PowerPoint presentations, originated in the office of a senior official at the MoD.
The documents relating to the Royal Navy’s Type 45 destroyer, HMS Defender, show that a mission described by the MoD as an “innocent passage through Ukrainian territorial waters”, with guns covered and the ship’s helicopter stowed in its hangar, was conducted in the expectation that Russia might respond aggressively.
Read more: BBC