Belgian King Philippe is to lead the centennial remembrance in Flanders Fields marking the first use of chemical weapons in warfare.

The monarch will go to the fields where Germany first used chlorine gas in its April 22, 1915, attack, to pay tribute to all victims of such arms over the past century.

Philippe will be joined in the ceremony, the highlight of two days of events, by the head of the Organization for the Prohibition of Chemical Weapons, which is still fighting the continued use of gas one century later.

Allies and Germany used so many chemical shells during the 1914-1918 war that ploughing farmers still come across them in old battlefields to this day. The rival armies launched 146 gas attacks in Belgium alone.