After authorities at first said early in the week that the leak represented no danger to people or the environment, they have since decided that precautionary measures are needed.

But the change of tune has fuelled local residents' fears that they were warned too late about contamination risks.

"We've been abandoned," complained 70-year-old Leopold Gravy, a retired school director in Fleurus, where the leak last weekend caused the most serious nuclear incident ever in Belgium.

Earlier police cars cruised Fleurus' streets giving the town's 20,000 residents instructions over loudspeakers about precautionary measures to take.

The town's population and residents of villages within a five-kilometre radius were asked not to eat fruit and vegetables from gardens or drink milk from neighbouring farms until further notice, said mayor Jean-Luc Borremans.

"Last Saturday we were told that (the leak) wasn't serious, Tuesday it was worse and today it's catastrophic," said Richard Charlier, a 54-year-old plumber who works in the area.

"The information has been bad and untruthful," said his colleague Roberto Carbini, 58.

Mayor Jean-Luc Borremans acknowledged: "The population is worried, it's normal. All things nuclear scare people.

"But I have confidence in the experts who tell me that it's without danger and that only precautionary measures are needed," he added. "There's been an incident and not an accident."

The incident occurred last weekend at the Institut des Radioelements, a laboratory which makes radioisotopes used in medical imaging and treating cancer.

Staff detected a leak of radioactive iodine in a ventilation chimney and alerted Belgium's Federal Nuclear Safety Control.

The agency gave the leak a rating of three out of seven on an international scale for nuclear incidents, making it the most serious ever detected in in the kingdom.

The lab halted production on Tuesday, but the agency said the same day that the leak did not represent a risk to residents in the area or the environment and did not recommend any steps be taken.

However, authorities changed their minds on Thursday evening after analysis of grass samples taken from the site suggested that radioactive iodine levels were higher than the first tests indicated.

The government decided during a meeting at its crisis centre late Thursday to call on residents to take precautionary measures while more tests were carried out.

"I'm thinking about moving," confided Philippe Hocq, a 40-year-old unemployed watch repairman, who said that nuclear experts had come to his mother's house, where he lives, to check for contamination.

"They took lettuce and pears and are supposed to contact us in several days with the results. But what's already gotten into our bodies? We don't know," he said.

The laboratory is the second biggest producer of medical radioisotopes in the world and it warned earlier this week that hospitals in several countries could face a shortage if its production remains halted for very long.

"It's a company that has a not-negligable humanitarian role so we have to keep cool heads," Borremans said.