Lambeth Palace announced today that the Most Reverend Justin Welby had been ill for several days and decided this morning that he was too unwell to speak at Canterbury Cathedral.
The head of the Church of England was expected to speak to worshippers about the famous World War One Christmas truce, in which British and German troops met in no man's land 100 years ago for a festive game of football.
According to a preview of his speech, his words were: "At Christmas 1914, soldiers took the risk, crossed a battleline and kindled an evening of friendship and football.
"It is the moment all have picked on this year, whether in adverts or sermons.
"The truce illustrates something of the heart of Christmas, whereby God sends his Son, that vulnerable sign of peace, to a weary war-torn world.
At Christmas 1914, soldiers took the risk, crossed a battleline and kindled an evening of friendship and football
"The problem is that the way it is told now it seems to end with a 'happy ever after'.
"Of course we like Christmas stories with happy endings: singing carols, swapping photos, shaking hands, sharing chocolate, but the following day the war continued with the same severity.
"Nothing had changed; it was a one-day wonder.
"That is not the world in which we live, truces are rare."
He was also due to urge worshippers not to reduce the story of Christmas to something "utterly remote, about lives entirely different, fictional, naive, tidy".
A Lambeth Palace spokesman said this morning: "The Archbishop of Canterbury, the Most Rev Justin Welby, is suffering from a severe cold and will, with great regret, no longer be preaching the sermon at Canterbury Cathedral this morning.
"The Dean of Canterbury, the Very Rev Robert Willis, will deliver a homily."