The Queen and senior royals will lead the nation in commemorating the Battle of the Somme today by attending services in both the UK and France.
At Westminster Abbey the Queen and Duke of Edinburgh will join the congregation for an evening commemoration - on the eve of the 100th anniversary of the start of the battle.
And in France the Duke and Duchess of Cambridge and Prince Harry will attend a vigil in memory of the fallen.
The services will mark the beginning of events across the UK and France marking the start of the battle on July 1 1916, a day that became the bloodiest in British military history with almost 20,000 dead.
By the end of the four-month battle in northern France, more than a million soldiers had been killed and wounded on both sides of the fighting.
The First World War would drag on for another two years.
After the Westminster Abbey service an overnight vigil will begin around the Grave of the Unknown Warrior - the first time the place of worship has hosted such an event since the peace vigils for the Cuban Missile Crisis more than 50 years ago.
The Dean of Westminster, the Very Reverend Dr John Hall, said: "The Battle of the Somme is remembered as one of the most devastating of the First World War.
"The sacrifice of many thousands of lives in a battle which ultimately failed to achieve any serious breakthrough stands as a reminder of the suffering and loss involved in the First World War.
"As we imagine the feelings of those preparing for battle, the vigil will allow us to reflect on the cruel effects of warfare and to pray for lasting peace and justice in the world. All will be welcome."
Other overnight events will also take place in Wales, Scotland and Northern Ireland.
William, Kate and Harry will attend events this evening at the Thiepval Memorial in France, where 70,000 British and Commonwealth soldiers with no known grave are commemorated.
The events include a climb to the top of a huge, newly renovated structure, which will be lit for the first time, to view the killing fields.
There is to be a military vigil and a meeting with representatives of nations involved in the battle.
On Friday they will be joined by the Prince of Wales, Duchess of Cornwall, Prime Minister and 10,000 members of the public, including hundreds of schoolchildren, chosen by ballot, for a service of commemoration.
Charles and Camilla will then attend other ceremonies for Northern Irish and Canadian victims of the battle at the nearby Ulster Tower and Beaumont-Hamel Newfoundland Memorial, respectively.
Camilla will also lay a wreath at the grave of her great-uncle, Captain Harry Cubitt, who was killed on the Somme in September 1916 while serving with the Coldstream Guards.
He was the eldest, and the first, to die of three brothers killed serving on the Western Front.
In the UK the nation will fall silent for two minutes during Friday morning to remember the sacrifices of all those who fought and lost their lives.