Hundreds of guests will attend a remembrance service in France marking the 100th anniversary of the final day of the Battle of the Somme.
A daily service has been poignantly hosted by the Royal British Legion at the Thiepval Memorial to The Missing to commemorate each day of the bloody four-and-a-half month offensive which started on July 1 1916.
Senior royals and politicians including the Prince of Wales, the Duke of Duchess of Cambridge and Prince Harry attended an international service at Thiepval in July marking the 100th anniversary of the battle's beginning.
A more low-key service will be held on Friday to commemorate the centenary of the final day of the battle, which was fought by 60 nations from across the British Empire and Europe across a 15-mile front in northern France.
Led by Bishop James Newcome, the Royal British Legion's national chaplain, guests will be welcomed by Britain's ambassador to France, Lord Llewellyn of Steep.
The British Army suffered almost 60,000 casualties on the first day of the battle alone, marking it out as the bloodiest day in British military history. And more than a million men were killed or wounded on both sides over the course of the 141 days.
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 high numbers of fatalities as troops headed into German machine gun fire.
The Rt Revd Nigel McCulloch, head of remembrance at the Royal British Legion, said those who fought had left an "enduring legacy" across the UK and Commonwealth.
He said: "Throughout the First World War centenary, the legion has led the nation in remembering the men whose sacrifice has come to symbolise the tragic scale and futility of modern industrialised warfare.
"The last day of the Somme is a moment to reflect on the collective sacrifice of all those who fought and fell in such tragic numbers between 1 July and 18 November 1916."
The Thiepval Memorial to the Missing is the largest Commonwealth War Graves Commission monument in the world, inscribed with the names of more than 72,000 soldiers whose bodies were never recovered or identified.
Earlier this month, it was announced that hundreds of harrowing first-hand accounts, mostly written experiences of the Tommies who went over the top on July 1 1916, would be made public for the first time.
Among those in the Imperial War Museum collection was that of Private John Kirkham, a 20-year-old in the 20th Battalion Manchester Regiment in 1916, who described attacking a German soldier with a "knobkerrie", a steel-headed club used in hand-to-hand fighting.
Another correspondent, Lance Corporal Charles Bartram, 23, who had been at the front with his mortar unit for a matter of weeks, described seeing his senior officer killed by a sniper in front of him as they waited to go "over the top".