Britain’s navy has launched a probe after official documents about one of its nuclear-powered submarines were reportedly found in the toilet of a pub in north-western England.

They carried details about the HMS Anson, a £1.3 billion (S$2.2 billion) “hunter killer” submarine, Britain’s The Sun newspaper reported.

The files were marked “official sensitive”.

The nuclear-powered attack vessel was built at a shipyard in the town of Barrow-in-Furness, in Cumbria, very close to the pub where the files were found.

The pub was said to be packed when the files were discovered on the floor of a cubicle, the newspaper said, quoting a source.

The navy said the documents contained no classified information – only simplistic designs of the systems on board and not 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.”

HMS Anson is 97m long, and is the fifth of a new class of attack submarines to join Britain’s naval fleet.

According to the British Ministry of Defence, these submarines are the “largest, most advanced and ‘most powerful’ attack submarine ever operated by the navy”.

The Anson is also said to be able to circumnavigate the globe without resurfacing.