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 BBC reported on Sunday 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 Britain said it was a making "an innocent passage" in accordance with international law.

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.

Britain's allies, including the United States, had been informed, 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 Quin.