Poet and novelist Siegfried Sassoon in May 1915.

Siegfried Sassoon’s war diaries – some still bearing traces of mud from the Somme – have been made available online, 100 years since Britain declared war on Germany.

About 4,100 pages of the war poet’s personal archive have been digitised for the Cambridge University Library project, which aims to highlight the horrors of World War I.

They include draft copies of his Soldier’s Declaration as well as poetry, prose and sketches.

The poems include previously unpublished material along with early drafts of some of his best-known works including an early version of The Dug-Out with an additional, excised verse.

The notebooks were small enough to be carried in the pocket of his army tunic, and he used them to draft poetry, make pencil or ink sketches, list members of his battalion and their fates, make notes on military briefings and draw diagrams of the trenches.

The Sassoon archive is a collection of towering importance

The images of them show mud from the trenches and spilled wax, presumably as he sat writing in his dug-out by candlelight.

Cambridge University librarian Anne Jarvis said: “The war diaries Sassoon kept on the Western Front and in Palestine are of the greatest significance, both nationally and internationally, and we are honoured to be able to make them available to everyone, anywhere in the world, on the 100th anniversary of World War I.

“From his Soldier’s Declaration to his eyewitness accounts of the first day of battle on the Somme, the Sassoon archive is a collection of towering importance, not just to historians, but to anyone seeking to understand the horror, bravery and futility of World War I as experienced by those on the front lines and in the trenches.”

Made up of 23 journals and two of his wartime poetry notebooks, much of the archive has until now been out of reach of the public and researchers due to the poor physical condition of the documents.

Only his official biographer, Max Egremont, has had access before.

Mud, possibly from the battlefield, on Sassoon’s war diaries. Photos: Cambridge University Library/PA Wire

The online archive covers the years 1915-1927 and 1931-1932, as well as two poetry notebooks from 1916-1918 containing rough drafts and copies of some of his best-known war poems.

Sassoon wrote in a small and legible hand, frequently using his notebooks from both ends.

John Wells, from the library, said: “The great array of activities, difficulties and dangers that faced him as a serving officer, and the recurring inspiration of his creative responses to his conditions, are represented in the range of uses to which he put these notebooks.

“Unlike edited, printed transcriptions, the digitisations allow the viewer to form a sense of the physical documents and to appreciate their unique nature as historical artefacts.”

The archive can be viewed at http://cudl.lib.cam.ac.uk .