One hundred years after the beginning of the Battle of Passchendaele, the tens of thousands of soldiers killed in one of the First World War's bloodiest campaigns are being remembered in a series commemorations.

Prime Minister Theresa May, the Prince of Wales and the Duke and Duchess of Cambridge will attend events in Ypres, Belgium on Sunday, including a military ceremony.

Prince Charles and Belgium's Queen Mathilde and King Albert will open a British Memorial Poppy Garden in Passchendaele Memorial Park.

More than 325,000 Allied troops and 260,000 German were killed in the three-month campaign officially known as the Third Battle of Ypres.

The infantry attack began on 31 July. Within a few days, heavy rainfall turned the already churned fields into a quagmire so deep men and horses drowned in it.

The Allied troops were led by Field Marshal Douglas Haig whose decision to continue with the fighting at Passchendaele after it was clear the offensive would make no real gains has since been widely criticised.

Winston Churchill later said Haig "wore down alike the manhood and the guns of the British army almost to destruction."

War poet Siegfried Sassoon wrote of the carnage: "I died in hell - they called it Passchendaele"

The battle ended on 6 November when the ravaged village of Passchendaele was captured by British and Canadian forces. Despite the months of fighting and the heavy casualty cost, the Allied forces had advanced just five miles.

The prime minister of the time, David Lloyd George, wrote in his memoirs: "Passchendaele was indeed one of the greatest disasters of the war... no soldier of any intelligence now defends this senseless campaign."