Official documents about a £1.3 billion Royal Navy “hunter killer” submarine were reportedly found in the toilets of a Wetherspoons in Cumbria.

The Sun reported that files carrying details about the HMS Anson were left in The Furness Railway in Cumbria.

A source told The Sun the pub was packed when the files, marked “official sensitive”, were discovered on the floor of a cubicle.

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 are described as the “largest, most advanced and most powerful attack submarines” ever used in the navy on its website.

The documents were said to have shown the inner workings of the submarine. 

However, The Royal Navy has said the papers were generic resources and did not contain any classified information.

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.”

A naval source added the files only contained simplistic designs of the systems on board, without revealing how they work.

The Furness Railway is a short distance from a BAE systems shipyard in Barrow-in-Furness, where the submarine has previously been pictured.