A secret network of tunnels along the White Cliffs of Dover that were used during the Second World War have opened to the public for the first time.

Constructed on the orders of then prime minister Winston Churchill to bolster Britain's coastal defences, Fan Bay Deep Shelter was equipped with gun batteries designed to prevent German ships moving freely in the English Channel.

Some 2m below the surface, the 325 sq m shelter is one of the deepest recovered of all time. Carved out of the chalk in just 100 days by Royal Engineers from the 172nd Tunnelling Company, the site accommodated and catered for up to 190 soldiers during wartime.

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There are 325sq m of tunnels,  23m below the surfaceNational Trust

Decommissioned in the 1950s and filled in during the 1970s, the labyrinth of tunnels had been hidden for the past 40 years until they were discovered by the National Trust.

The charity found the maze of tunnels underneath land it purchased in 2012. Since then, a team of more than 50 National Trust volunteers, two archaeologists, two mine consultants, two engineers and a geologist have spent nearly two years excavating and preparing them to be opened to the public.

Now, from 20 July visitors will be able to be taken down on torch-lit tours into the heart of the White Cliffs as part of a new visitor experience that serves as a time-capsule of the Second World War.

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A sound mirror, a precursor to radar, being tested during the First World WarNational Trust

Jon Barker, visitor experience manager at the White Cliffs, said in a statement: "This rediscovered piece of the country's Second World War heritage is a truly remarkable find.

"There's been no public access to the tunnels for more than 40 years and so they remain much as they were when they were abandoned. We've carried out extensive conservation work to preserve both the natural decay and authentic atmosphere of the space. After spending 18 months working below ground to literally uncover its long-kept secrets, we hope visitors will be as intrigued by the tunnels as we are."

Visitors to the tunnels will also be able to see two historic sound mirrors which form part of the site. Uncovered by the National Trust in November 2014, the two rare First World War mirrors were of the first early warning devices invented in the UK, designed before the advent of radar to give advanced notice of approaching enemy aircraft.

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The White Cliffs of Dover are synonymous with Britain's defences against enemy attackNational Trust

"Fan Bay Deep Shelter has a powerful effect, telling the human story behind Dover's crucial role during the First and Second World Wars. The White Cliffs are forever linked to our country's defences and the tunnels, with their graffiti-covered walls, are an emotive time capsule from this period," Barker says.

45-minute guided tours of Fan Bay Deep Shelter are now on-sale on the National Trust website, costing £10 per adult and £5 for 12-16 year olds.