The Queen and senior royals will lead Britain in remembrance to mark the centenary of the Battle of the Somme.
Events across the UK and in France will commemorate 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 time the battle in northern France finished four months later, more than a million soldiers had been killed and wounded on both sides of the fighting.
World War I would drag on for another two years.
At Westminster Abbey in London, the Queen and Duke of Edinburgh will join the congregation for an evening vigil on Thursday, the eve of the anniversary of the start of the battle. Other overnight events will take place in Wales, Scotland and Northern Ireland.
The Duke and Duchess of Cambridge and Prince Harry will attend evening events at the monumental 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 the 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 Duke and Duchess of Cornwall 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 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.
Beginning on July 1, 1916, the Battle of the Somme was intended to achieve a decisive victory for the British and French against Germany's forces.
The British Army was forced to play a larger than intended role after the German attack on the French at Verdun in February 1916.
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 2000 men of the the 1st and 2nd Bradford Pals, both part of the West Yorkshire Regiment, suffered 1770 casualties in the first hour of the offensive as they attacked the heavily fortified village of Serre.
