Theresa May, Prince William mark centenary of key WWI battle

Britain's Prime Minister Theresa May and Prince William will take part in a ceremony marking the centenary of the Battle of Amiens, which marked the beginning of the end of World War I. (Photo: AFP/Oli Scarff)

Families of soldiers who took part in the lightning Allied advance, which smashed German defences and morale, precipitating the end of the war, have travelled to Amiens from across the world for the ceremony in the city's cathedral.

Representatives of the Australian, British, Canadian, French and US governments will also commemorate the tens of thousands of troops killed in the four days of fighting, along with former German president Joachim Gauck.

The Battle of Amiens sounded the start of the Hundred Days Offensive on the Western Front, which led to the Armistice in November 1918.

It marked a shift away from trench to armoured warfare, with the Allies deploying hundreds of tanks to push deep into German lines on what German General Erich Ludendorff called "the black day of the German army" in the war.

May's visit is her second to France in under a week, coming days after she held talks with President Emmanuel Macron at his Mediterranean holiday retreat over her Brexit plan.

The British premier, who is under pressure to win allies on the continent for her divorce strategy, will read aloud at the ceremony from the war memoirs of Britain's wartime leader Lloyd George.

Prince William will also address the proceedings, and the two will also meet with soldiers' families, including descendants of the crack Canadian and Australian troops who led the Allies into battle.

Macron, a native of Amiens, is not himself scheduled to attend.

SHOCK TACTICS

The ceremony is the latest in a busy year of World War I commemorations.

Yet despite the Allies landing a decisive blow in Amiens, the battle never gained the same place in the popular imagination as longer, bloodier World War I clashes such as the Battles of the Somme or Verdun.

By the summer of 1918 American troops were pouring into France and the Allies had drastically boosted their firepower, emboldening them to strike back at flagging German forces on the Western Front.

Supreme allied commander General Ferdinand Foch convinced the French, British and Americans to mount a stealth attack around the city of Amiens, a key rail and logistics hub.

When guns began pounding German positions before dawn on Aug 8, the Germans were caught off guard.

Marooned in dense fog, thousands of dazed soldiers surrendered to the Allies, who dug a gaping, 12km hole into German lines, backed by some 600 tanks and 2,000 warplanes.

The stunned German army never recovered. By early September it was in retreat and two months later the war was over.