High levels of toxic chemicals were found in groundwater tests at a new facility scheduled to replace Tokyo's Tsukiji fish market -- the world's largest -- clouding the costly relocation plan, news reports said on Saturday.

The controversial project has been delayed over fears about toxic contamination, which also impacted the 2020 Olympics to be hosted by the Japanese capital.

The megacity's new governor, Yuriko Koike, has said she would postpone the move originally set for November last year, as she awaits final groundwater testing results at the new site, a former gas plant.

The relocation plan has been marred by problems, including the discovery that contractors had inexplicably failed to fill in a basement at the new site with clean soil as a buffer against underground pollution.

Results of the final testing showed that levels of toxic materials, including benzene, detected in underground water at the new site were above nationally set limits, the Yomiuri Shimbun and Jiji Press said.

It came after the previous test also showed high levels of mercury were found inside the facility's basement.

Plans to uproot the more than 80-year-old market, a popular tourist attraction, have been in the works for years, with advocates citing the need for upgraded technology.

The local government paid a whopping 86 billion yen ($750 million) in cleanup costs.

But Koike, a former TV anchorwoman elected last year as Tokyo's first female governor, has pledged to reconsider the plan.

"What we have to uphold is food security," Koike said Saturday, according to Jiji Press. "We may have to study further."

Koike, however, has not said if she would consider scrapping the relocation altogether if the test results are bad.

She has also questioned the 588 billion yen in relocation costs -- more than one-third higher than earlier estimates -- to put the market on a site several kilometres away and build a modern facility about 40 percent larger with state-of-the-art refrigeration.

Postponing Tsukiji's move has also prompted Koike to suspend plans to construct a tunnel under the current Tsukiji market leading to an athletes' village for the 2020 Games because it could not be built in time.

Meanwhile, Tsukiji's wholesalers have voiced frustration over the delay, saying that postponing the move will cost them millions of dollars a month.