A highly radioactive substance, emitting in some places radiation 100 times the permitted amount, has been discovered in Switzerland, local media reported, saying the hazard was known about for 18 months.

Le Matin Dimanche and Sonntags Zeitung reported on Sunday that federal, regional and local officials decided not to reveal the discovery of radium in an old dump in the town of Bienne in case it scared the 50,000 residents.

"120kg of radioactive waste was obtained after sorting. We measured doses of several hundred microsieverts at the source," Daniel Dauwalder, a spokesman for the Swiss federal office for public health, told Le Matin Dimanche.

In certain places, measurements of 300 microsieverts per hour were taken, more than 100 times the permitted amount for an old dump, the newspapers reported.

Anyone exposed for three hours would absorb the equivalent of a year's permitted dose.

The waste came from a paint used by the watch-making industry to illuminate the numbers on watch faces.

The substance, which has been banned since 1963 due to its radioactive nature, was discovered when roadworks were started at the site.

The OFSP judged the risk to public health "weak", although Sonntags Zeitung said that tests on the water table would begin next month.

The president of the federal commission in charge of monitoring radiation, which was not informed of the incident, said the various authorities had made a "mistake".

"This will all come back to bite us and it is much more difficult to stay credible and win back the public's trust," Francois Bochud told Le Matin Dimanche.