SECRET nuclear sub plans were found in a toilet cubicle at a Wetherspoons pub.

Classified files on £1.3billion HMS Anson had been dropped in The Furness Railway in Barrow, Cumbria.

A source said: “It was lucky a Russian spy didn’t find them.”

The files showed the inner workings of the ­torpedo-loaded vessel.

Key detail on HMS Anson’s hydraulics, which control torpedo hatches, steering and buoyancy, were in the dossier.

It was found in the boozer with a Royal Navy lanyard from the new £1.3bn vessel.

A source said pub was packed when the papers, marked Official Sensitive, were found on Saturday night.

Government guidance says ­that information marked “Sensitive” must only be shared on “genuine need to know” and could have damaging ­consequences if lost, stolen or ­published.

The source said: “It was quite a lively night.

“The pub was full of people from the docks — military and civilian.

“I went into the toilet and the plans were lying on the floor of the cubicle with the lanyard.

“Anyone could have found them.

“It was lucky it wasn’t some deep cover Russian spy.”

A naval source said the ­documents were part of a reference manual that is readily available on board.

Anybody who had worked on the submarine would have used the manual.

Official sensitive documents are one level below Secret.

Former sub captain, Commander Ryan Ramsay said: “It looks like someone has taken the pages off the boat to study.

“They are part of a book that cover all the systems on a sub.

“When they do their basic submarine qualification they have to walk round the boat to demonstrate they know all the systems.

“It is good to see their commitment to training, but the pub is probably the wrong place.”

The boozer is a five-minute walk from the BAE Systems’ shipyard where workers build Astute class hunter-killer subs.

HMS Anson is the fifth such vessel.

It launched in February and is now based at His Majesty’s Naval Base, Clyde, Scotland, for sea trials.

It is armed with Spearfish torpedoes and Tomahawk land attack missiles.

The Navy describes the sub as the “cutting edge of the UK’s military capabilities” and the most advanced hunter-killer in the world.

HMS Anson’s nuclear reactors mean it can go 25 years without refuelling.

And it can circumnavigate the globe without resurfacing.

Air and water are made onboard so the only limit to deployment is how much food it carries for crew.

Last night a Navy spokesperson told The Sun the papers were “generic training documents.”

They added: “However, we take all security matters extremely seriously and will investigate the circumstances of their discovery.”

Two years ago The Sun told how classified defence documents were found in a soggy heap at a bus stop in Kent.

They contained details of covert missions for UK special forces in Afghanistan after the US withdrawal.

An investigation found papers were removed from the MoD by top civil servant Angus Lapsley, who briefly had his security clearances revoked.

But the married dad-of-two was later promoted to Nato’s Assistant Secretary General responsible for defence policy and planning.