Classified defence documents containing details about HMS Defender and the military have been found at a bus stop, prompting an investigation from the Ministry of Defence (MoD). The loss of the sensitive information was described “as embarrassing as it is worrying for ministers” by Labour, who are seeking reassurances that national security has not been undermined. The MoD said an employee reported the loss of the documents last week, which were discovered by a member of the public in a soggy heap behind a bus stop in Kent early on Tuesday morning, the BBC reported. The papers included one set of documents that 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. A member of the public, who wanted to remain anonymous, contacted the organisation when they found the 50 pages of classified information, which included emails and PowerPoint presentations. Britain was unexpectedly embroiled in a diplomatic and military dispute with Russia on Wednesday after the Royal Navy Type 45 destroyer, HMS Defender, briefly passed through territorial waters off the coast of the disputed territory of Crimea. The documents confirm 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 the Kremlin might respond aggressively. A series of slides showed two alternative routes, one described as “a safe and professional direct transit from Odessa to Batumi”, including a short stretch through a maritime traffic-management system, known as a TSS, close to the south-west tip of Crimea. This option would “provide an opportunity to engage with the Ukrainian government … in what the UK recognises as Ukrainian territorial waters”, one slide concluded. Three potential Russian responses were outlined, from “safe and professional” to “neither safe nor professional”. On Wednesday, Russia said warning shots had been fired, and that a jet had dropped four bombs in the path of the British destroyer to force it to change course. It emerged that cannon had been fired at a safe distance behind the British warship, while there was no evidence to support the latter claim. Labour’s shadow defence secretary, John Healey, said it was vital that the MoD’s internal inquiry immediately establishes how highly classified documents were taken out of the department in the first place and then left. “The loss of classified documents that cover a wide range of vital areas of national security is as embarrassing as it is worrying for ministers,” 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,” he added. The MoD said HMS Defender “conducted innocent passage through Ukrainian territorial waters in accordance with international law” and that all potential factors were considered when making operational decisions. An MoD spokesperson said in a statement: “As the public would expect, the Ministry of Defence plans carefully. As a matter of routine, that includes analysing all the potential factors affecting operational decisions. HMS Defender conducted innocent passage through Ukrainian territorial waters in accordance with international law. “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.”