Official documents about a £1.3bn Royal Navy “hunter killer” submarine were reportedly found in the toilets of a Wetherspoons pub.
The Sun reported that files carrying details about the HMS Anson were left in The Furness Railway in Cumbria.
The documents showed the inner workings of the submarine and were used by submariners learning how to isolate and depressurise elements of its system.
The Royal Navy has said the papers were generic resources and did not contain any classified information.
A source told The Sun the pub was packed when the files, marked “official sensitive”, were discovered on the floor of a cubicle.
The Furness Railway is a short distance from a BAE systems shipyard in Barrow-in-Furness, where the submarine has previously been pictured.
HMS Anson is the fifth of the new Astute-class attack submarines to join the Royal Navy fleet.
The vessels are capable of firing tomahawk missiles and described as the “largest, most advanced and most powerful attack submarines” ever used in the navy on its website.
A naval source said: “These documents enable submariners and contractors to understand how systems interact. They do not detail how they work, just that they exist.”
They added that the files only contained simplistic designs of the systems on board, without revealing how they work.
A Royal Navy spokesman said: “These are generic training documents that carry no classified information. However, we take all security matters extremely seriously and will investigate the circumstances of their discovery.”