The high levels of contamination were detected in December in one of the inspection wells at the Toyosu market, said Michio Yasuma, an infrastructure coordination official with the Tokyo Metropolitan Government.

However, Yasuma said the facility, slated to open in October on the site of a former gas plant, was safe and groundwater contamination was not getting worse.

“Air quality data taken above ground, where businesses will operate, show that, scientifically, it is safe,” Yasuma told AFP.

“What needs to be monitored is the trend of groundwater samples – whether it is going up or down, or staying the same. Our surveys show groundwater quality remains largely stable,” he said.

The world-famous Tsukiji market is due to close in October to make way for a transport hub for use during the Tokyo 2020 Olympic Games.

The market, near the Ginza shopping district, has long attracted tourists who flock to its traditional pre-dawn tuna auctions.

However, the market, which dates from 1935, is old and crumbling and does not conform to modern safety and sanitary standards.

Tourists walking its crowded alleys often remark on the stench of raw fish and crumbling walls.

The new market at Toyosu, around three kilometres (two miles) to the east, offers modern sanitation and refrigeration.

But the move has been repeatedly delayed for various reasons including the discovery of soil contamination.

There is no plan to use groundwater at Toyosu and officials have covered soil with concrete and other materials.

But the Tokyo government is regularly monitoring air and groundwater samples for benzene and arsenic.

The well containing the extremely contaminated groundwater samples was located in a paved parking lot of the market, official documents show.