|A wounded soldier is carried through mud during the Battle of Passchendaele|
It is estimated to have claimed the lives of 275,000 British Empire troops in the West Flanders region of northern Belgium.
The French lost around 8,500 soldiers, while estimates for German casualties range from 217,000 to 260,000.
The battlefields turned into liquid mud.
The 'Last Tommy', Harry Patch, who died aged 111 in July 2009, was among those who fought in the offensive.
:: Passchendaele - 100 years on from WW1's muddy carnage
|The Tyne Cot military cemetery, where the bodies of soldiers of the Commonwealth army lie|
At 7pm UK time, a Last Post ceremony will begin at the Menin Gate, which is etched with the names of thousands of missing soldiers.
There will also be events at the Tyne Cot military cemetery on Monday.
|On 31 July 2017 it will be 100 years since the start of the Battle of Passchendaele, between the Britain Empire and its allies and the German Empire, in the First World War|
"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."