Senior politicians and royals will gather in western Belgium to commemorate 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 31 July, 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 Ms 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.

The killing fields of the First World War Show all 17 1 /17 The killing fields of the First World War The killing fields of the First World War Tyne Cot cemetery in a Flanders field near Ypres: most of the soldiers buried here are British Stefan Boness/Panos Pictures The killing fields of the First World War A recently discovered bunker in a Flanders field near Kemmel, Belgium Stefan Boness/Panos Pictures The killing fields of the First World War The landscape of Mesen near Ypres Stefan Boness/Panos Pictures The killing fields of the First World War Gravestones in a snow-covered Flanders field cemetery in Langemark Stefan Boness/Panos Pictures The killing fields of the First World War Old munition shells at Hill 62 Stefan Boness/Panos Pictures The killing fields of the First World War The Hill 60 crater was formed on 7 June 1917, when British miners and soldiers dug a tunnel under the German front post and detonated 45,000 kilos of explosives Stefan Boness/Panos Pictures The killing fields of the First World War A bunker at Langemark Stefan Boness/Panos Pictures The killing fields of the First World War Crosses with poppies in memory of the fallen soldiers at Hill 62 Stefan Boness/Panos Pictures The killing fields of the First World War The names of German soldiers at a war cemetery near Ypres Stefan Boness/Panos Pictures The killing fields of the First World War The A19 motorway cuts through Flanders fields Stefan Boness/Panos Pictures The killing fields of the First World War Signs for Ypres, Passchendaele and Langemark Stefan Boness/Panos Pictures The killing fields of the First World War A former battlefield near Wieltje Stefan Boness/Panos Pictures The killing fields of the First World War A golf course next to the Oak Dump cemetery Stefan Boness/Panos Pictures The killing fields of the First World War An Irish memorial cuts through a Flanders field near Mesen; Zandvoorde Stefan Boness/Panos Pictures The killing fields of the First World War A German bunker near Ypres Stefan Boness/Panos Pictures The killing fields of the First World War Trees along the N313 near Admiral's Road Stefan Boness/Panos Pictures The killing fields of the First World War A shrine at Totenmuehle Stefan Boness/Panos Pictures

"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.