Officials in the western German city of Koblenz have ordered the evacuation of about 45,000 people as part of preparations to defuse a World War Two bomb found in the Rhine River.

The 1.8 ton bomb, believed to have been dropped by Britain's Royal Air Force more than six decades ago, was uncovered after water levels in the Rhine dropped as a result of a prolonged dry spell. Two smaller bombs were discovered in the mud nearby.

Everyone living within a two-kilometer radius of the bomb site has been ordered to leave the area.

Experts are expected defuse the bomb on Sunday when all road and rail connections to Koblenz are suspended.

The area around the bomb was surrounded by sandbags Saturday and the water remaining inside the sandbagged area will be pumped out Sunday morning. Once the bomb site is dry, the process of deactivating the explosives can begin. This is expected Sunday afternoon.

Allied forces bombed Germany especially between 1943 and 1945 to hasten the surrender of its then Nazi leadership. Decades after the war unexploded ordinance is still being found. But the evacuation in Koblenz is the biggest bomb-related evacuation in Germany's post-war history. Nearly half the population of Koblenz is affected, including seven nursing homes, two hospitals and a prison.