Construction workers stumbled upon the 500-kilogramme (1,100-pound) bomb on the right bank of the Rhine river on Monday evening, city officials said.

It was dropped by American air forces during the war when the western German city was heavily bombed.

Experts will start the defusing operation at midday (1100 GMT), following the evacuation of nearby offices, including those of broadcaster RTL, and the Cologne opera.

Just 15 residents live in the exclusion zone, the city said in a statement.

The bomb was found close to the Hohenzollern rail and pedestrian bridge that leads to Cologne's famous Dom cathedral and central train station on the opposite bank.

German rail operator Deutsche Bahn announced that because of the exclusion zone no trains would be stopping at Cologne's smaller Messe/Deutz station from 0800 GMT, though the central station remained open.

The closure of the crucial Hohenzollern bridge for the duration of the bomb disposal efforts would lead to train delays and cancellations throughout the day, it warned, affecting mainly long-distance journeys.

The city of Cologne said the airspace above the danger zone would also be closed from midday, while local river traffic would be halted in one of the world's busiest waterways.

The discovery of World War II bombs is not uncommon in Germany.

Earlier this month, some 14,000 people had to leave their homes in Dortmund after two unexploded bombs were found in the city centre.

In 2017, the discovery of a 1.4-tonne bomb in Frankfurt prompted the evacuation of 65,000 people -- the largest such operation since the end of the war in Europe in 1945.

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