More than half a million troops - including 325,000 Allied soldiers - died in the battle, officially known as the Third Battle of Ypres, that lasted for more than three months in 1917.
"One hundred years on the name Passchendaele remains synonymous with the inhuman conditions and bloody ferocity of one of the First World War's most horrific battles", Clair Popplewell said.
The British royals will join Belgian king and queen Philippe and Mathilde at the ceremony.
Prince Charles paid tribute to soldiers alongside 4,000 of their relatives at the Tyne Cot cemetery in Ypres.
Prince William stood with Belgium's King Philippe during a ceremony honoring the dead. "Today, said with gravity, the future king of England in power, the Menin Gate lists the almost 54,000 names of men who had not returned, the missing who have no known grave".
"Members of our families; our regiments; our nations; all sacrificed everything for the lives we live today", he said.
They watched as thousands of paper poppy petals, one for every name on the Menin Gate, fluttered to earth from the roof above the gathered crowd.
The area is the largest Commonwealth burial ground in the world, with 11,971 servicemen laid to rest and remembered there, 8,373 of whom remain unidentified. The last post, a military tune played to remember the dead, was played to finish the service.
The Allied campaign, fought by British and Commonwealth forces from July to November 1917 in mud-caked battlefields, barely moved the front line against the Germans.
It featured performances from Dame Helen Mirren and journalist Ian Hislop, who introduced a sketch from his First World War play The Wipers Times.