I have seen the Tomb of The Unknown Warrior in Westminster Abbey many times, but it wasn’t until I stopped to study the inscription a couple of years ago that I fully understood what it represented. “Beneath this stone rests the body of a British warrior unknown by name or rank brought from France to lie among the most illustrious of the land and buried here on Armistice Day 11 Nov, 1920.”

Those simple words truly resonated. I suppose I’d always thought it was symbolic, like the Cenotaph. Embarrassingly, the penny dropped. There really is the body of an unknown soldier under there. I feel I should have known this.

So how did an anonymous corpse from the First World War come to be buried among the kings? Why did vast crowds turn out to witness the interment of an unidentified body in a box? And how does the Unknown Warrior still exert such a powerful hold on us today?

These are the questions I have sought to answer in my new book and stage show, The Unknown Warrior.

Reverend David Railton, an Oxford-educated army chaplain and vicar of Folkestone is at the heart of this incredible story. His job was to carry a Bible rather than a rifle and offer spiritual support to the war-ravaged troops as best he could.

Padres like Railton conducted countless burials, many using a treasured Union Jack to cover a body during the brief service. Necessarily short because of the ongoing battles – and the sheer number of the dead – as soldiers gathered to say farewell, they were deeply aware they might soon be lying beneath the ‘padre’s flag’ themselves.

As I wrote in Saturday’s Express, I was stunned when I began researching my new book to learn that 526,816 British and Commonwealth soldiers have no known resting place. Of those, 338,955 were never buried at all, while 187,861 have graves but have never been identified. Their bodies were blown to pieces by shellfire or lost in the choking mud of the trenches as the fighting raged back and forward.

Repatriations of the dead were banned by the Government as impractical and unfair. So there were no funerals where families and communities could come together and lay their sons and fathers to rest.

Even before the Somme offensive from July 1 until November 18, 1916, in which the British Empire suffered 420,000 casualties for an advance of eight miles, David Railton had witnessed death and destruction on a scale no man should ever have to see.

One particular incident in early 1916 was seared into his mind. “We had just laid to rest the mortal remains of a comrade. I went to a billet in front of Erkingham [sic], near Armentieres,” he recalled. “At the back of the billet was a small garden and a grave. At the head of the grave there stood a rough cross of white wood. On the cross was written in deep black pencilled letters ‘An unknown British Soldier’ and in brackets underneath ‘of the Black Watch’… How that grave caused me to think! How I wondered! How I longed to see his folk! But, who was he, and who were they?”

So were sown the early seeds of a concept that would eventually become an enduring national symbol: a tomb for all the missing with no known graves.

As the war rumbled on, Railton privately nurtured his idea. What must have terrified him was the thought that, if he failed to persuade the right people at the first attempt, the whole idea risked being dismissed.

Finally, on August 13, 1920, Railton, who won the Military Cross for bravery despite his supposedly ‘safer’ role away from the fighting, composed the letter he had been thinking about for nearly four years.

He chose to write to the Right Reverend Bishop Herbert Ryle, Dean of Westminster, who had the ear of both the King and the Prime Minister. Distilling all his experience, fervour and powers of persuasion, he asked him to consider the possibility of burying in Westminster Abbey the body of “one of our unknown comrades”, to represent the hundreds of thousands of fallen who had no identifiable grave.

Railton even dared suggest his personal battle-stained Union Jack might be used at such a burial. To be fair, he was not the only person to have come up with the idea of burying the body of an unknown soldier as a symbol of national loss. Two newspapers – including the Daily Express – had put forward early proposals for some form of burial of a soldier with no name.

But there can be no doubt it was his concept which ignited the process that would become the focus of the nation on the second anniversary of the end of the war.

The King was initially sceptical but the idea gained traction in high places and, on October 19, 1920 – just three weeks before the November 11 services – Ryle wrote to Padre Railton with the good news. Behind the scenes, things were now moving quickly.

A government committee headed by Foreign Secretary Lord Curzon had already been tasked with organising the grand ceremonial parade on Armistice Day and the unveiling of Lutyens’s permanent Cenotaph in Whitehall. Now it was instructed to conjure up a state funeral as monumental as if the man in the coffin had been a garlanded national leader, not an anonymous soldier.

But with only three weeks to go before the ceremony, who was going to decide who this Warrior would be?

At the end of the First World War, the quiet town of Saint-Pol-sur-Ternoise, 50 miles south of Calais in northern France, had become the HQ of the Directorate of Graves Registration and Enquiries. From here Brigadier General Louis Wyatt was in charge of the ongoing campaign to exhume, identify and formally rebury the countless dead still spread across the Western Front.

Years later, General Wyatt outlined his initial thoughts upon receiving his instructions for the choosing of the Unknown Warrior; that the body “must be a British Soldier, and that there could be no means of him being identified”, and that he should come from one of the four big battle areas: Aisne, Somme, Arras, Ypres.

Thus on November 8, 1920, four field ambulances carrying men equipped with shovels and sacks clattered to a halt outside four cemeteries across the Western Front. It must have been strange. The exhumation parties, trained to identify bodies, were now doing the exact opposite.Accordingly, four sets of remains were brought to the chapel in Saint-Pol to await their destiny.

“I selected one [body],” Wyatt wrote years later, without expanding upon how he did so. The chosen remains, presumably still in a sack, were lifted off the stretcher and placed inside a simple pine coffin which waited by the altar. The other three would be returned to the earth. The chosen body was to be guarded overnight, transported in state to London to be buried among poets, artists and monarchs. His final resting place already awaited him, prepared that day in the nave of Westminster Abbey. Was he an officer or an enlisted man? Which regiment? From where did he hail? Was he rich or poor? Was someone still searching for him? Little is known. Exactly what David Railton had intended. This truly was an Unknown Warrior and his final journey was about to begin.

Amidst great ceremony, the body was taken to Boulogne harbour, across the sea to Dover and then by train to Victoria station in London. As dawn broke over London on November 11, 1920, two weary Grenadier Guardsmen stood steady on the station’s platform eight.

The pale glow creeping through the vast canopy above their heads offered the first inkling that their vigil guarding the coffin of the Unknown Warrior was almost at an end.

The casket was placed onto a gun carriage and drawn by six black horses through immense and silent crowds to the Cenotaph in Whitehall which was unveiled by the King at 11am. The great procession then moved towards Westminster Abbey, the King taking up his role as chief mourner, walking stiffly behind the Unknown Warrior’s carriage.

In his wake came the princes, the bearer party, hundreds of servicemen, six abreast, with thousands of troops and veterans following behind.

More than 20,000 applications had been received for the roughly 1,600 places that had been made available at the abbey.

Perhaps most poignantly of all, the newspapers had come across a 12-year-old boy who had written his own plea to the authorities, ending his letter with the enduringly resonant thought shared by so many: “The man in the coffin might be my daddy.”

Inside, alongside the 99 war widows who had lost their husband and every single one of their sons, there was an honour guard of those awarded the Victoria Cross and other awards for valour in the face of the enemy. The funeral procession made a slow and graceful turn in front of the iron gates outside the North Door of the abbey.

The massed bands wheeled away into a side street and fell silent as the bearer party halted. The leather straps on the gun carriage were released and the heavy, zinc-lined oak coffin was lifted onto their shoulders. It was time. The bearer party carried their dead comrade slowly through two ranks of helmeted policemen, Railton’s Union Jack draped over it, marching with measured steps from the November sunlight into the gloom of the abbey’s interior.

Halting, they laid the coffin on the wooden timbers placed across the grave, which, as the New York Times pointed out, stood “in the pathway of kings, for not a monarch can ever again go up to the altar to be crowned, but he must step over the grave of the man who died that his kingdom might endure”.

The brief service that followed was, according to The Times, “the most beautiful, the most touching and the most impressive that, in all its long eventful story, this island has ever seen”. A fitting conclusion then to the more than half a million stories which had lain unfinished since the end of the war.

Finally, a gleaming silver shell filled with the soil taken from the battlefields was handed to the King, who sprinkled a small amount over the coffin with his fingers, before reverently tipping the rest into the grave as the Dean spoke: “Earth to earth, ashes to ashes, dust to dust, in sure and certain hope of resurrection to eternal life.”

So an odyssey which had begun on one of the flayed battlefields of France had at last reached its conclusion.

No one could have foretold how the Unknown Warrior would capture the imagination – a seven-mile queue of mourners, standing four-deep, waited to pay their respects. Veterans on crutches, faces disfigured by shrapnel, stood alongside children, parents and uncles. One little boy who stooped to place a posy made even the policemen standing guard blink back tears. “Oh look, Mummy,” he cried. “What a lovely garden my daddy’s got!”

A week later, the abbey’s great doors were finally locked against the procession of frozen mourners still hoping to be allowed inside. One hundred sandbags of soil from the battlefields of France and Flanders were now emptied over the coffin then a temporary slab of marble sealed the tomb.

On it was chiselled the gilded inscription: “A British warrior who fell in the Great War 1914–1918 for King and Country. Greater love hath no man than this.”

• Edited extract by Matt Nixson from The Unknown Warrior by John Nichol (Simon & Schuster, £22), published September 26. To pre-order, visit expressbookshop.com or call Express Bookshop on 020 3176 3832. Free UK P&P on orders over £25. John is touring the book nationwide from October 4 to November 7, tickets and information via johnnichollive.com