Police ordered the establishment of a six-day security perimeter around the Zaluzi plant about 100 kilometres (60 miles) northwest of Prague after workers found the ordinance at a remote part of the factory's site.

The 250-kilogramme (550-pound) British aerial bomb was equipped with a unique chemical trigger to delay its explosion, which police said made it particularly dangerous.

"The danger and rarity of this discovery lies in the triggering mechanism that can be set for up to 144 hours, or six days," a police statement said.

Polish petrol giant Orlen, which controls the plant, said in a statement that the discovery forced it to shut the plant's oil refinery "until further notice".

Besides the 1.5-square-kilometre perimeter, the Czech police also ordered 600 people to evacuate.

Czech explosives experts were consulting their "German colleagues, who have more experience in finding aerial bombs with chemical time-delay triggers", the police statement added.

The factory, which served the Nazi war effort during the German occupation of then Czechoslovakia, was the target of Allied bombings in 1944.