A member of the public who wished to remain anonymous contacted the BBC after finding 50 pages of classified information behind a Kent bus stop on Tuesday morning.
The documents were “found in a soggy heap”, the BBC reported.
The Ministry of Defence (MoD) confirmed the incident on Sunday, saying an employee had reported losing the documents last week.
“The Ministry of Defence was informed last week of an incident in which sensitive defence papers were recovered by a member of the public,” an MoD spokesperson said in a statement shared with The Independent.
“The department takes the security of information extremely seriously and an investigation has been launched,” they said.
It is unclear whether the individual who left the documents behind at the bus stop could face any action.
“The employee concerned reported the loss at the time,” the spokesperson said, adding: “It would be inappropriate to comment further.”
According to the BBC, the documents included information on a mission described by the MoD as an “innocent passage through Ukrainian territorial waters” with guns covered and the ship’s helicopter stowed away in its hangar.
The documents suggested the mission would be conducted in the “expectation that Russia might respond aggressively”, the BBC reported.
Indeed, HMS Defender sparked a reaction from Russian forces on Wednesday after it made its way through waters south of the Crimean peninsula, which the Kremlin illegally annexed from Ukraine in 2014.
The ship had been headed to the Indo-Pacific region with the UK Carrier Strike Group, but it was announced earlier this month that it would be splitting with the group to carry out its “own set of missions” in the Black Sea.
Defence secretary Ben Wallace has said Moscow’s response on Wednesday, which saw several aircraft follow the ship at heights as low as 500 feet, was “neither safe nor professional”.
Russia has also said that warning shots were fired by their vessels. However, the UK government has said that it was only a routine “gunnery exercise” that took place.
The MoD said that HMS Defender “conducted innocent passage through Ukrainian territorial waters in accordance with international law” in a statement appearing to echo the contents of the misplaced documents.
Shadow defence secretary John Healey has called the incident “as embarrassing as it is worrying for ministers”.
“It’s vital the internal inquiry launched by the secretary of state establishes immediately how highly classified documents were taken out of the Ministry of Defence in the first place and then left in this manner,” he told the Press Association.
“Ultimately ministers must be able to confirm to the public that national security has not been undermined, that no military or security operations have been affected and that the appropriate procedures are in place to ensure nothing like this happens again,” Mr Healey said.
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