The nation will fall silent to honour thousands of soldiers killed in the Battle of the Somme, 100 years after its bloody beginning.
The Prince of Wales will lead senior royals and politicians including Prime Minister David Cameron and French president Francois Hollande in remembering the fallen during a ceremony at the Thiepval Memorial to the Missing on the former battlefield in Picardy.
The nation will come to a halt at 7.28am for two minutes before 7.30am, the time the "Tommies" and their French allies launched the attack on the German lines.
The first day of the Battle of the Somme became the bloodiest in British military history with more than 57,000 casualties recorded - of these 19,240 were fatalities.
Among the worst hit were the "pals" battalions - volunteer units of limited fighting experience.
Many were told to walk slowly across no man's land, resulting in massive numbers of dead as they headed straight into German machine gun fire.
The Duke of Cambridge, accompanied by the Duchess and Prince Harry, spoke at the start of a military vigil at Thiepval on Thursday.
In an address written by Birdsong novelist Sebastian Faulks, William highlighted the almost 60,000 British and Commonwealth casualties of July 1 1916, the bloodiest day in the history of the British Army.
William told the assembled guests: "We lost the flower of a generation; and in the years to come it sometimes seemed that with them a sense of vital optimism had disappeared forever from British life.
"It was in many ways the saddest day in the long story of our nation.
"Tonight we think of them as they nerved themselves for what lay ahead. We acknowledge the failures of European governments, including our own, to prevent the catastrophe of world war."
The Queen had earlier led a vigil at Westminster Abbey, echoed across the UK and Ireland.
During the service, prayers were said for the First World War dead and hymns were sung by the congregation who included descendants of the men who fought at the Somme.
The Queen symbolically touched a wreath that was placed at the Grave of the Unknown Warrior and the first watch took up its place - one person, head bowed as a mark of respect, at each corner of the tomb, with their numbers drawn throughout the all-night vigil from UK and Commonwealth military personnel and members of community groups representing those involved in the battle.
During the service the Last Post was played with a bugle that had been used at the Somme.
Welsh Guardsman Lance Sergeant Stuart Laing, 39, sounded the moving tune from the Lantern Tower - the first time music had been performed from the eaves of the Abbey.
On Thursday night Mr Cameron said the commemoration allowed people "to reflect on the sacrifice not just of the thousands of British and Commonwealth troops who gave their lives, but of the men on all sides who did not return home".
He said: "It is an opportunity to think about the impact of the devastation felt by communities across all of the nations involved, which left mothers without sons, wives without husbands and children without fathers.
"The young men who left our shores believed in the cause for which they fought and we honour their memory.
"But today is also a chance to stand as friends with the representatives of all the countries who are here today.
"This event and the Thiepval monument itself bear testament to a solemn pledge - those who died here will never be forgotten."
Mr Cameron, Mr Hollande and the royals will be joined by heads of state and representatives from the nations who fought in the battle, as well as French, British and Irish schoolchildren, descendants of those who fought and an audience of 10,000 members of the public.