Ministers will be forced to explain how sensitive defence documents containing details about HMS Defender’s controversial deployment in the Black Sea came to be found at a bus stop.
The Ministry of Defence (MoD) said an employee reported the loss of the documents last week, and the department has launched an investigation.
A member of the public, who wanted to remain anonymous, contacted the BBC when they found 50 pages of classified information in a soggy heap behind a bus stop in Kent.
The papers, which were found on Tuesday last week, included one set of documents which discussed the potential Russian reaction to HMS Defender travelling through Ukrainian waters off the Crimea coast the following day, while another laid out plans for a possible UK military presence in Afghanistan.
Shadow defence secretary John Healey has been granted an urgent question in the Commons to force ministers to set out how the security blunder occurred.
A spokesperson for the MoD said: “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.”
Mr Healey called the incident “as embarrassing as it is worrying for ministers”.
HMS Defender caused a diplomatic incident when it travelled through waters south of the Crimean peninsula, which Russia unofficially annexed from Ukraine in 2014.
Moscow responded by having several aircraft shadowing the ship at varying heights, the lowest being approximately 500ft, which Defence Secretary Ben Wallace said was “neither safe nor professional”.
Russia also claimed that warning shots were fired by their vessels at the destroyer, but this was dismissed by the UK Government which said only that a routine “gunnery exercise” took place.
Moscow has threatened to retaliate if the incident is repeated, but Prime Minister Boris Johnson insisted the warship was “entirely right” to make the trip from Odessa in Ukraine to Georgia as an internationally recognised transit route.
The MoD said HMS Defender “conducted innocent passage through Ukrainian territorial waters in accordance with international law”.