Almost three decades after the Chernobyl disaster thousands of kilometres away, a jump in radioactivity levels is making Norwegian reindeer meat unfit for consumption, a government agency said Thursday.

Levels of caesium-137 -- a radioactive isotope that is released into the atmosphere during a nuclear power plant accident -- has reached up to 8,200 becquerel per kilogram in reindeer this year in central Norway, the region most affected by the 1986 radioactive cloud that left thousands dead.

"This is way above the limit for slaughtering the animals," Inger Margrethe Eikelmann, a researcher at the Norwegian Radiation Protection Authority, told AFP.

The 3,000 becquerel threshold is higher than the 1,500 to 2,500 becquerel average found in the animals two years ago.

As a result, the traditional slaughter of hundreds of reindeer in late September did not take place.

"We have seen a decline of caesium in the ecosystem for many years and we thought that this year the reindeers would be under the authorised levels," said Eikelmann.

The rebound in radioactivity levels is due to warm, humid weather this summer that has been conducive to the growth of gypsy mushrooms, a favourite among grazing livestock such as reindeer and sheep.

The mushrooms absorb the nutrients found in the upper layers of the soil where most of the caesium-137 particles can be found.

However, the readings may offer little solace for Santa's helpers as the radioactivity levels in the animals will halve in two or three weeks if they refrain from eating mushrooms, which disappear naturally as soon as the first frost appears, according to Eikelmann.

If their diet fails to improve in the wild, their owners could keep them in an enclosed pasture, feed them properly and shoot them in November or December, she said.