The Duke and Duchess of Cambridge join thousands of relatives of those who fought at the Battle of Passchendaele in a gathering which marks 100 years since it began.
The Prince of Wales, Prime Minister Theresa May and Defence Secretary Sir Michael Fallon will also attend events in and around Ypres, Belgium, to commemorate the centenary which cost tens of thousands of British and Commonwealth lives.
On Sunday, a ceremony will take place at the Menin Gate in Ypres, which is etched with the names of thousands of missing soldiers.
Events will also be held at the Tyne Cot military cemetery on Monday.
Sir Michael said: "These services provide us with the time 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.
"This was a battle which touched communities across Europe and it is a privilege to be here in Belgium to stand as friends with the representatives of all the countries who took part in the battle - friends who continue to be strong allies."
The British and Commonwealth attacks were fought near Ypres between July 31 and November 10 1917, in battlefields that turned to liquid mud and were summed up in poet Siegfried Sassoon's line "I died in hell, they called it Passchendaele".
More than half a million troops - 325,000 Allied soldiers and 260,000 Germans - died in the battle, officially known as the Third Battle of Ypres, in the West Flanders region of northern Belgium in 1917.
Among those to fight in the battle was the "Last Tommy" Harry Patch, who died aged 111 in July 2009.