Some of the files contained details about HMS Defender (Image: Getty)
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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.
Documents relating to British plans for Afghanistan came with the warning 'secret UK eyes only'
The Government said it had launched an investigation into the incident, according to the BBC.
The broadcaster reports that documents relating to Afghanistan discuss a US request for British help in certain areas and whether UK special forces will remain in Afghanistan after the Nato withdrawal.
It reportedly warns: "Any UK footprint in Afghanistan that persists… is assessed to be vulnerable to targeting by a complex network of actors."
It adds that "the option to withdraw completely remains".
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Northern Ireland Secretary Brandon Lewis said he was aware of the papers found at a bus stop.
He told Sky News: "It’s something none of us want to see and shouldn’t be able to happen.
"It was properly reported at the time into the Ministry of Defence, there’s obviously an internal investigation into that situation as well, and I’ve got to allow… the MOD to work through that investigation to understand exactly how this managed to occur.”
Briefing notes from the UK-US Defence Dialogue were also part of the pack
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.
According to the BBC, the papers show the decision to send HMS Defender close to Crimea was a decision made by the British Government to show support for Ukraine.
The files also reportedly include updates on arms exports and notes from last Monday's UK-US Defence Dialogue.
Russia released images purportedly showing it firing warning shots at HMS Defender
(Image: FSB Russia)
It also suggests officials should use the meeting to see whether Joe Biden's administration is prepared to back its pledge to consult more with allies.
On Wednesday, witnesses claim HMS Defender 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.
'Polite' Russians asked Royal Navy to 'please' retreat six times before 'shooting'
Boris Johnson hits back at Russia over HMS Defender 'barefaced lies' claims
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.
Mr Johnson has come under pressure to reveal whether he personally authorised the Royal Navy destroyer to use the route.
The secret documents reportedly show the UK considered Russia's reaction before sending a ship into Ukrainian waters
He said: "These are a matter for the MoD but if you want my view I think it was wholly appropriate to use international waters, and by the way the important point is that we don't recognise the Russian annexation of Crimea, this is part of a sovereign Ukrainian territory, it was entirely right that we should vindicate the law and pursue freedom of navigation in the way that we did, take the shortest route between two points, and that's what we did."
"I think it was very important for the carrier strike group to do what they're going to do around the world, in partnership with 40 other countries on manoeuvres, sticking up for our values, sticking up for what we believe in.
"That includes democracy, human rights, equalities, but also the rule of law and freedom of navigation. 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 go from A to B."