The UK's defence ministry has apologised after top secret documents assessing the movements of a Royal Navy warship in the Black Sea were found at a bus stop.
Junior defence minister Jeremy Quin told parliament the government was "deeply sorry" for the lapse, which he blamed on a senior ministry official.
One of the documents, which were found by a member of the public in Kent, southeast England, and handed to the BBC, was marked "Secret: UK Eyes Only".
The broadcaster reported that the cache of papers discussed the possible Russian response to the Royal Navy's HMS Defender travelling through Ukrainian waters off Crimea.
Russia last week said it fired warning shots at the destroyer for violating its territorial waters, straining further diplomatic ties between London and Moscow.
But the UK said it was a making "an innocent passage" in accordance with international law.
Mr Quin told MPs the mislaid documents were now back with the ministry and the individual's access to sensitive material was suspended, pending an investigation.
The unnamed employee reported the loss, he said, adding: "I don't want to prejudge the investigation but it appears this was a mistake by that individual."
Also among the documents were plans for a possible continued British military presence in Afghanistan after the end of US-led NATO operations later this year.
The UK's allies, including the United States, had been informed, Mr Quin said, adding he had "no evidence" that the safety of personnel had been compromised.
Several MPs in Prime Minister Boris Johnson's Conservative Party noted repeated security breaches at the Ministry of Defence in the last six months.
"We have no records of documents being mislaid at the above 'Secret' level in the last 18 months. But clearly this should never have happened," said Mr Quin.
UK defence minister and top military commanders self-isolating
The UK Defence Secretary and top military commanders are self-isolating after the chief of the armed forces tested positive for Covid-19 last week.
Ben Wallace and the heads of the Royal Navy, Royal Air Force and strategic command were alerted by the National Health Service test and trace app to stay at home for a period of ten days after coming into close contact with chief of defence General Nick Carter, the ministry of defence confirmed.
Head of the army, General Mark Carleton-Smith, and General Carter's deputy have also spent the weekend in quarantine while they await the result of PCR tests after also attending the meeting but maintaining a greater distance from the commander, the Daily Telegraph reported.
General Carter, 62, held the meeting at the Defence Academy at Shrivenham, Oxfordshire, on Thursday with Mr Wallace, vice-chair of the defence staff admiral Tim Fraser and General Carter.
Also present were an admiral, air force chief, and another commanding army general.
Social distancing measures are said to have been adhered to during the meeting, but the men still came into contact with General Carter, who tested positive the day after the meeting after appearing at a history festival on Friday.
Military business will be conducted remotely while the commanders self-isolate during the period.
An MoD spokesperson said: "The chief of the defence staff has tested positive during routine Covid-19 checks. Colleagues who were in a senior meeting with him last week, including the secretary of state, are self-isolating in line with government guidelines."
Additional reporting PA
