Samples of grass taken next to the National Institute for Radioactive Elements near the southern city of Charleroi had shown higher levels of radioactive iodine than the first tests, an interior ministry crisis group said here.
The Belgian federal nuclear watchdog gave the incident a three on an international scale of nuclear events that runs to seven, making it the most serious ever in Belgium.
Late Thursday it warned people living near the laboratory not to eat lettuce from their gardens and told farmers not to feed their cattle with grass from the fields there.
Production of radioisotopes was stopped on Monday after an abnormally high level of iodine was detected over the weekend in a ventilation chimney.
Hospitals in several countries could face a shortage of medical radioisotopes used for imaging and treating cancer after the laboratory halted production following the iodine leak, the institute said Wednesday.