The Ministry of Defence (MoD) said the employee concerned with the loss of documents reported it last week, and the department has launched an investigation.
A member of the public, who wanted to remain anonymous, contacted the BBC when they found 50 pages of classified information in a soggy heap behind a bus stop in Kent early on Tuesday morning.
The papers included one set of documents that discussed the potential Russian reaction to HMS Defender’s travel through Ukrainian waters off the Crimea coast on Wednesday, according to the BBC, while another laid out plans for a possible UK military presence in Afghanistan.
The MoD said HMS Defender “conducted innocent passage through Ukrainian territorial waters in accordance with international law” and that “all potential factors” are considered when making “operational decisions”.
A spokesperson said: “As the public would expect, the Ministry of Defence plans carefully.
“As a matter of routine, that includes analysing all the potential factors affecting operational decisions.
“HMS Defender conducted innocent passage through Ukrainian territorial waters in accordance with international law.”
They added: “The Ministry of Defence was informed last week of an incident in which sensitive defence papers were recovered by a member of the public.
“The department takes the security of information extremely seriously and an investigation has been launched.
“The employee concerned reported the loss at the time.
“It would be inappropriate to comment further.”
HMS Defender is part of the UK Carrier Strike Group heading to the Indo-Pacific region.
However, it was announced earlier this month that it would be temporarily breaking away from the group to carry out its “own set of missions” in the Black Sea.
The Type 45 destroyer caused a clash with Russian forces on Wednesday when it travelled through waters south of the Crimea peninsula, which Russia unofficially annexed from Ukraine in 2014, in a move that was not recognised by international powers.
Moscow responded by having several aircraft shadowing the ship at varying heights, the lowest being approximately 500 feet – which Defence Secretary Ben Wallace said was “neither safe nor professional”.