Classified defense documents containing details about the U.K. military vessel HMS Defender and the British military have been found at bus stop in the country, the BBC reported on June 27.
The British government said the employee concerned with the loss of documents reported it last week, and an investigation has been launched.
A member of the public, who wanted to remain anonymous, contacted the BBC after finding 50 pages of classified information in a soggy heap behind a bus stop in Kent on June 22.
The papers included one set of documents which discussed the potential Russian reaction to HMS Defender's transit through Ukrainian waters off the Crimea coast on June 23, according to the BBC, while another laid out plans for a possible British military presence in Afghanistan.
The Defense Ministry said that HMS Defender "conducted innocent passage through Ukrainian territorial waters in accordance with international law.”
A spokesperson said: "As the public would expect, the Ministry of Defense plans carefully.
The spokesperson added: "The Ministry of Defense was informed last week of an incident in which sensitive defense papers were recovered by a member of the public.
"The department takes the security of information extremely seriously and an investigation has been launched.
Britain's main opposition Labour Party said the discovery of the documents by a member of the public was "as embarrassing as it is worrying for ministers".
Labour's defense policy chief John Healey said ministers needed to confirm that national security had not been undermined or security operations affected and that "procedures are in place to ensure nothing like this happens again."
The HMS Defender is part of the U.K. Carrier Strike Group currently heading to the Indo-Pacific region.
Russia claimed that its vessels on June 23 fired warning shots and a military plane dropped bombs in the path of the British destroyer HMS Defender to force it to change course from the area near the Crimean city of Sevastopol.
Britain’s Defense Ministry denied the HMS Defender had been fired upon, saying that Russia was carrying out a previously announced "gunnery exercise" in the area.
Russia seized Ukraine's Crimea region in March 2014, sending in troops and staging a referendum denounced as illegitimate by at least 100 countries.
Western countries have imposed sanctions on Russia over the annexation of Crimea, the treatment of jailed opposition activist Aleksei Navalny, election interference, and cyberattacks on U.S. infrastructure blamed on Russian hackers.
British Foreign Secretary Dominic Raab reiterated during a visit to Singapore that "no shots were fired" as the British ship was "conducting innocent passage through Ukrainian territorial waters.”
British Cabinet member George Eustice said on June 24 in a televised interview that his country's warships could sail again through the disputed waters around Crimea because Russia's seizure of Crimea was illegal.