Authorities in the German city of Cologne have evacuated three hospitals and thousands of homes after the discovery of an unexploded second world war bomb during construction work on a new medical campus.

The 1,000kg US aerial bomb, equipped with a front and rear impact detonator, is due to be defused on Friday.

A complex evacuation procedure had been in the planning since excavation work on the site began about six months ago, owing to well-founded fears that unexploded ordnance would be discovered there.