The Prince of Wales will lead senior royals and politicians in commemorating the centenary of one of the bloodiest offensives of the First World War.
Charles, the Duke and Duchess of Cambridge and Prime Minister Theresa May will be among those attending a ceremony marking 100 years since the Battle of Passchendaele.
More than 100 days of fighting in the summer and autumn of 1917, starting on July 31, left more than half a million men dead or injured on both sides.
Monday's commemoration centres on the Tyne Cot cemetery near Ypres in Belgium, the largest Commonwealth burial ground in the world with 11,971 servicemen buried or remembered there - with 8,373 of them identified.
The British royals will join Belgian king and queen Philippe and Mathilde at the ceremony.
It comes after William and Kate joined Mrs May to represent Britain at the Menin Gate and a later show in the Gross Markt square.
William spoke as the daily Last Post was played at the towering edifice, inscribed with the names of the missing from three years of hard fighting around Ypres a century ago.
Watched by some 200 descendants of those who fought, he said: "During the First World War Britain and Belgium stood shoulder to shoulder.
"One hundred years on, we still stand together, gathering as so many do every night, in remembrance of that sacrifice."
Sunday's poignant Last Post was the 30,752nd time it has been played since 1928.
The towering Menin Gate in the Belgian town is covered with the names of 54,391 British dead who have no known grave, according to the Commonwealth War Graves Commission.
In just over three months of conflict there were more than half a million casualties - 325,000 Allied soldiers and 260,000 to 400,000 Germans - in the Belgian battlefields.
Later, dignitaries and hundreds of guests watched the show, led by Dame Helen Mirren, which included testimony from soldiers projected on to the walls of the Cloth Hall.