The 14-line poem, To a Citizen-Soldier, was found by Jean Moorcroft Wilson, the poet's biographer, in papers belonging to Sassoon's earliest publisher, Edward Dent, which are held at Cambridge University Library.

Sassoon wrote the poem in the days following the death on 18 March 1916 of David Thomas, a younger fellow-officer with whom he had fallen in love.

Sassoon: Reassessment of his career?

It also followed shortly after the death in Gallipoli of the poet's younger brother, Hamo.

The poem, published for the first time in the Daily Telegraph, also followed the start of conscription and the beginning of Sassoon's experiences in a front-line trench.

In the poem, Sassoon displays his anger at the death of his beloved friend and berates a conscript - known as a "citizen-soldier" - for moaning about conditions on the Western Front:

"For such as you there is One end, one bias; scatheless to slip back To frowsty parlours of familiar bliss, And there forget these writhings on the rack, To boast and wrangle, loaf and sleep and kiss And grow ignobly senile, gorged and slack."

Literary experts say the scorn for the conscript in the poem may have embarrassed Sassoon and could explain why he never published it.

They also predict the discovery will lead to a reassessment of the poet's career.

Miss Moorcroft Wilson said: "Nobody would make any arguments for this being a great poem, but it is historically very significant.