Classified documents containing details about HMS Defender and the British Armed Forces have been discovered at a bus stop in Kent.

The sensitive defence papers were found by a member of the public on Tuesday – it has not been confirmed exactly where in the county they were found.

One set of documents is said to discuss the likely Russian reaction to the ship's voyage through Ukrainian waters off the Crimea coast on Wednesday, the BBC reported.

Another weighs up plans for a possible "UK military presence in Afghanistan" after the US-led NATO operation there ends.

An investigation has been launched after the sodden papers were found having been sat out in the rain.

The person who found them, who wished to remain anonymous, reportedly contacted the BBC after realising they were sensitive.

The Ministry of Defence says an employee had reported the loss at the time and it would be "inappropriate to comment further”.

"There will be a full investigation and when things like this happen we've got to ensure it can't happen again."

A spokesman for the department 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."

On HMS Defender, it added: “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 type 45 destroyer is part of the UK Carrier Strike Group and is currently carrying out missions in the Black Sea, according to the Royal Navy's website.

“HMS Defender conducted innocent passage through Ukrainian territorial waters in accordance with international law.”

It was involved in an altercation with Russian naval vessels earlier this week over its journey through territorial waters.

On Wednesday more than 20 Russian aircraft and two coastguard ships shadowed the warship. Warning shots were reportedly fired at the destroyer.

According to the BBC's reports, three potential Russian responses were outlined in the found papers, from "safe and professional" to "neither safe nor professional".

An alternative route was also considered, which would have meant HMS Defender avoided the contested waters.

Most of the papers are allegedly marked "official sensitive" – a relatively low level of classification.

But the BBC reports that one document addressed to the defence secretary Ben Wallace’s private secretary was labelled “Secret UK Eyes Only”.

It apparently outlines highly sensitive recommendations related to the UK's military presence in Afghanistan, following the conclusion of Nato’s current operation.

The broadcaster says it decided not to report certain details which could endanger the security of British and other personnel.

Speaking on the Andrew Marr Show this morning, Northern Ireland Secretary Brandon Lewis said: "It is a serious breach.

"I've spoken to the Defence Secretary and they are taking forward an internal investigation."

The cabinet member added: "To an extent processes were followed and it was reported by an individual as soon as they were aware they had left those documents.

"There will be a full investigation and when things like this happen - human error or otherwise - we've got to ensure it can't happen again."

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