OSAKA -- Greenpeace reported Tuesday that two ships on their way to Japan from Britain to collect tainted mixed uranium-plutonium (MOX) fuel and return it to England have passed through the Panama Canal.

According to the environmental group, the ships passed through the canal Monday.

"At the current rate of speed, we expect them to cross the Pacific and arrive in Japanese waters around June 6," said Shaun Burnie, a Greenpeace activist who tracks MOX and plutonium shipments.

The two vessels, the Pacific Pintail and the Pacific Teal, were sent by the British government and are on their way to Takahama, Fukui Prefecture, where they will retrieve a shipment of MOX fuel that was originally delivered in fall 1999.

It was discovered after the original shipment was delivered that quality control data had been falsified and Kansai Electric Power Co., under tremendous public pressure, agreed not to use the fuel. The British government agreed to take back the fuel at its own expense.

Burnie said it is unclear whether the ships will go directly to Takahama or stop in a commercial port in Japan to be refitted and wait out the World Cup soccer tournament.

"It seems hard to believe that Japan would attempt to move a shipment of MOX in the middle of the World Cup, when security concerns are heightened," he said. "It makes more sense to wait until after the matches finish at the end of June."

Kepco officials refused to comment on the Greenpeace reports, saying only that the ships, which left the U.K. in late April, would take between one and two months to reach Japan.