Secret plans for one of Britain's most fearsome deterrents against the Soviet threat have been found... in a charity shop.
Detailed drawings of a Trafalgar class nuclear submarine would have made a valuable prize for any Russian secret agent during the Cold War.
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But now they are being sold at a Barnardo's in north Wales.
Stella Parker, who works at the Porthmadog shop, discovered the plans while going through a suitcase full of old books which had been donated.
The drawings would have been a valuable discovery for a Cold War era spy
She said: "All the lining of the suitcase was ripped, as I went through the case I noticed this paper behind the lining. When I took it out I could see it was plans of a submarine.
"At first I thought it was the technical drawings because they looked like blueprints."
Mrs Parker took the map - which measures approximately 6ft by 3ft - to David Dickinson when the celebrity antiques dealer was filming his Real Deal television show in nearby Penygroes.
HMS Trafalgar was launched in 1981 and decommissioned in 2009
He made an offer - suggesting it might be worth between £30 and £40 - but she turned him down.
She said: "He told me it was the map used by the men to help find themselves around the sub. If they had been the real blueprints they would have been worth a lot more."Stella has rejected an offer from antiques expert David Dickinson for the drawings
Unfortunately, she never got around to taking it to auction and the super-sized poster stayed in a backroom of the shop because they were too big to display.
She said: "Doing some research on Google, I think they are the plans to HMS Trafalgar, which was the only submarine in the Trafalgar-class to have a bladed propeller."
It's believed the drawings belonged to an engineer for shipbuilders Vickers
The ship, which was launched in 1981, served in the Afghanistan War and became the first Royal Navy submarine to launch Tomahawk cruise missiles against Taliban forces.
In 1996 the sub gained notoriety after running aground off the Isle of Skye during a training exercise - causing £5million worth of damage - the second time it had suffered a mishap.The discovery was made in a suitcase of books given to Barnardo's in Porthmadog
The sub was finally decommissioned in 2009 and the plans - which appear to have belonged to engineer WG Clouter, a public relations manager for shipbuilders Vickers - have since been declassified.
Offers for the plans are being taken from Monday, March 13.