Prime Minister David Cameron has hailed the sacrifice of hundreds of thousands of soldiers killed and wounded at the Battle of the Somme ahead of the 100th anniversary of the catastrophic slaughter.
Mr Cameron will join dignitaries including French president Francois Hollande and the Duke and Duchess of Cambridge at the Thiepval Memorial to the Missing in northern France on Friday, exactly a century on from the bloodiest day in the history of the British Army.
It follows a night-long vigil, led by the Queen at Westminster Abbey and echoed across the UK and Ireland, which ends before 7.30am, the time the Tommies went over the top in Picardy on July 1 1916.
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, including Prince Harry, 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.
It is taking place at Sir Edwin Lutyens Thiepval monument, which bears the names of more than 72,000 British and South African soldiers who died in the Somme but whose bodies were never found.
The service will use hymns, readings and music reflecting the cultural and military impact of the site to help tell the story of the Somme.