The National Archives is set to release more than 150 secret spy files on individuals and organisations involved in spying during the First World War.
Photos, letters and interrogation reports are among the documents to be released online by the Archives. They contain details about spies operating across the globe, including the notorious female spy Mata Hari and Swallows and Amazons author Arthur Ransome.
Soldiers who had family members who served in World War I have spoken about how moved they were to be involved in the procession of sacred soil through the capital.
Trooper Russell Jarman's great grandfather served with the Royal Horse Artillery during the First World War.
"During the battle he went into No Man's Land to save the gun and horses that were caught in the shelling" he said.
"He brought the horses back to safety but was fatally wounded in the process. He was awarded a posthumous medal for bravery."
"He was the inspiration for me joining the cavalry. I work with horses every day, and every day think of him and what he must have gone through in Flanders. Riding with the gun carriage today was an emotional moment for me."
Kris Peeters, the Prime Minister of Flanders has paid tribute to the fallen at a special ceremony to bring soil from 70 World War I battlefield to London.
"We will never forget the sacrifices that were made to help us regain our freedom" he said.
"We will never forget about those four years, not only because it is a testimony of our friendship but also because it reminds us that we have to remain vigilant, even though we have escaped war for many decades now we must never forget how quickly things can change".