Hospitals and homes have been evacuated in the German city of Cologne after an unexploded World War II bomb was uncovered during construction work for a new medical facility.

The 1,000kg US aerial bomb will be detonated in a “controlled manner” at the building site later today after upwards of 7,000 people were forced to leave the Merheim district.

The explosive device was discovered equipped with both a front and rear impact detonator and a specialist team has been drafted in to manage the operation.

The evacuation of medical staff, patients and residents was supported by police, firefighters and hundreds of volunteers, who conducted door-to-door checks to clear the area.

The event has been described by local authorities as the biggest and most extensive evacuation procedure since the wartime era.

Work on the site began six months ago and, as part of the construction planning procedure, a detailed emergency evacuation strategy had been developed.

The forward-planning was largely due to concerns that such an explosion device may be found during the building works as Cologne was among one of the most-bombed cities during WWII.

Traffic and public transport were diverted away from the area as part of the plan and patients were transferred to other hospitals using more than 300 ambulances.

Residents have been sheltered in a school building just outside the 500-metre evacuation zone, which has also been established as a contact point for the community affected.

In an update issued on Friday afternoon, city authorities reported the bomb disposal unit was only able to defuse one of the two impact detonators.

As a result, the city announced the 2,000lb American bomb would be “detonated in a controlled manner”.

A subsequent update reassured residents that “all precautions” were being taken to ensure the operation was conducted safely.

Despite the change of plan, the exclusion zone has remained unchanged and residents are awaiting news of when they will be able to return to their homes.