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 tomorrow commemorate the start of the battle in 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 local time, the eve of the anniversary of the start of the battle. Other 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.
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.
THE SOMME IN NUMBERS
7 - days of artillery bombardment of the German lines before the battle started, aiming to cut the barbed wire and destroy trench defences and artillery
1,500,000 - artillery shells fired by the Allies in that week, to little overall effect
57,470 - British casualties on the first day
19,240 - British first-day casualties who died
60 - percentage of British officers involved on the first day who were killed
141 - days the battle lasted, from July 1 to November 18
419,654 - official number of British dead, missing or wounded
1,300,000 - approximate number of casualties on both sides
10 - km that British soldiers had advanced by the end of the battle
49 - Victoria Crosses awarded for valour during the battle
150,000 - graves in the area cared for in more than 250 military and 150 civilian cemeteries in the Somme area by the the Commonwealth War Graves Commission
72,000 - number of British and Commonwealth soldiers who died at the Somme with no known graves and whose names are recorded on the British memorial at Thiepval.
SOME WHO FOUGHT:
Adolf Hitler - The Nazi dictator was injured fighting for the German empire. Over the years there has been speculation that he suffered a wound to his genitals as well as the leg wound suffered while serving with a Bavarian unit, which gave rise to the legend that he only had one testicle.
Ralph Vaughan Williams - The composer, whose work The Lark Ascending is frequently voted Britain's most popular piece of classical music, enlisted as a private in the Royal Army Medical Corps on New Year's Eve 1914, the same year he produced the work. He was aged 42.
Otto Frank - Anne Frank's father was the only member of the family to survive the Holocaust. Born in Frankfurt he was drafted into the German Army in 1915 and served on the Western Front for the rest of the war, earning promotion to lieutenant. He moved the family from Germany to Amsterdam in 1933 after Hitler's rise to power.
Harold Macmillan - The British Conservative prime minister from 1957 to 1963 was an officer in the Grenadier Guards who was wounded twice during the Somme. He spent the rest of the war recovering and was left permanently affected.
JRR Tolkien: The Hobbit and Lord of the Rings author was an officer in the 11th Battalion, Lancashire Fusiliers. Serving in the difficult northern sector of the Somme battlefield, Tolkien's health eventually suffered. He contracted trench fever at the end of October 1916 and was then sent back to England.
Siegfried Sassoon - As a second lieutenant with the 1st Battalion, Royal Welsh Fusiliers, war poet Sassoon witnessed the carnage of July 1 weeks after earning a Military Cross in a daring operation to rescue a soldier in No Man's Land.
Robert Graves - A friend of Sassoon and a fellow war poet who served in the same unit, Graves was wounded in eight places on July 20 in a shell explosion. He was so badly hurt that his family were told he was dead and it was announced in the Times. He described his wartime experiences in Farewell To All That in 1929 and eventually died in 1985, aged 90.
Edmund Blunden: A poet contemprary of Sassoon and Graves, Blunden was physically uninjured by his war service but suffered from "shell shock", now known as Post Traumatic Stress Disorder, for the rest of his life. He described the Somme in works including Thiepval Wood.