Top secret Ministry of Defence documents revealing details about HMS Defender and the British military were found at a bus stop.
The papers, which comprise almost 50 pages, were reportedly found in a wet pile behind a bus stop in Kent on Tuesday morning.
They cover a range of subjects, including the expected Russian reaction to the Royal Navy destroyer HMS Defender passing through Ukrainian waters off the coast of Crimea on Wednesday.
Another document relates to UK plans for a potential continued military presence in Afghanistan after Nato’s withdrawal, the BBC reports.
A Military of Defence spokesman told the broadcaster that a worker had reported the loss of secret defence papers.
The Government said it had launched an investigation into the incident, according to the BBC.
The documents are said to reveal that the MoD thought Russia may react aggressively before HMS Defender sailed through Ukrainian waters off the coast of Crimea, which was illegally annexed by Russia in 2014.
The papers reportedly show that the mission, called ‘Op Ditroite’, was discussed at a high level as late as Monday.
On Wednesday, witnesses claim the Type 45 destroyer was buzzed by Russian military jets and the sound of naval gunfire could be heard as it sailed from Odessa in Ukraine to Georgia.
But the UK government dismissed Russia’s claims that it opened fire on the vessel.
Moscow accused the UK of telling “barefaced lies” and warned the incident will have serious consequences.
Asked whether the allegation, made by Russian foreign ministry spokeswoman Maria Zakharova, was correct, Boris Johnson said: “Well, they’re the bear.
“That’s not my information and my understanding is that the carrier strike group proceeded in the way you would expect through international waters and in accordance with the law.”
Russia claimed that four bombs were dropped in the path of the destroyer – but the Ministry of Defence disputes this.