Sensitive defence documents containing details about HMS Defender and the military have reportedly been found by a member of the public at a bus stop.
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 found 50 pages of classified information in a soggy heap behind a bus stop in Kent early on Tuesday morning.
The person, who wants to remain anonymous, then passed the files to the BBC.
The papers are said to include one set of documents which discussed the potential Russian reaction to HMS Defender's travel through Ukrainian waters off the Crimea coast on Wednesday, and another which laid out plans for a possible UK military presence in Afghanistan.
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."
On Wednesday, HMS Defender sailed close to the Crimean peninsular in the Black Sea, a move the UK says was in accordance with international law, but Russia claims violated its borders.
Russia says it chased the ship out of its waters and fired warning shots at it - something the UK denies.
Russia annexed Crimea from Ukraine in 2014, and the UK recognises the occupation of the region as "illegal".
The papers reportedly outline how HMS Defender's journey was being considered at the top levels of government as late as Monday.
And three potential response from Russia were discussed - from "safe and professional" to "neither safe nor professional".
Most of the papers were marked "official sensitive" - a fairly low level classification.
The documents also contained information addressed to Defence Secretary Ben Wallace's private secretary marked "Secret UK Eyes Only".
Information about the UK's military presence in Afghanistan following the winding down of the NATO presence in the region was contained in these papers.
Details about the UK's special forces were apparently contained in this part of the documents.
Speaking on Thursday, Boris Johnson said it was "not my information" when asked if the Russians had fired warning shots.
"My understanding is that the Carrier Strike Group proceeded in a way you would expect through international waters and in accordance with the law," said the prime minister.
He added: "We don't recognise the Russian annexation of Crimea - it was illegal. These are Ukrainian waters and it was entirely right to use them to get from A to B."