Sensitive defence documents containing details about HMS Defender and the military have been found by a member of the public at a bus stop, according to reports.

The Ministry of Defence (MoD) said the employee concerned with the loss of documents reported it 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 early on Tuesday morning.

The papers included one set of documents which discussed the potential Russian reaction to HMS Defender’s travel through Ukrainian waters off the Crimea coast on Wednesday, according to the BBC, while another laid out plans for a possible UK military presence in Afghanistan.

The department takes the security of information extremely seriously and an investigation has been launched

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.”

Shadow defence secretary John Healey 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 said.

“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.”

HMS Defender is part of the UK Carrier Strike Group currently heading to the Indo-Pacific region.

However, it was announced earlier this month that it would be temporarily breaking away from the group to carry out its “own set of missions” in the Black Sea.

The Type 45 destroyer caused a clash with Russian forces on Wednesday when it travelled through waters south of the Crimea peninsula, which Russia unofficially annexed from Ukraine in 2014, in a move which was not recognised by international powers.

Moscow responded by having several aircraft shadowing the ship at varying heights, the lowest being approximately 500 feet – 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 assertion 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, while 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 that HMS Defender “conducted innocent passage through Ukrainian territorial waters in accordance with international law” and that “all potential factors” are considered when making “operational decisions”.