The huge radioactive water leak discovered at a storage tank last week at the Fukushima No. 1 nuclear plant was likely caused by deteriorating rubber seams and distortions that emerged after the tank was reassembled, Tokyo Electric Power Co. says.

Tepco said Saturday that the temporary tank, which held water used to cool the melted fuel in three of the plant’s shattered reactors, was moved and reassembled after it began sinking two years ago amid subsidence at the site.

On Aug. 19, Tepco revealed that 300 tons of the tainted water had vanished from the tank, marking the fifth and worst leak there since the March 2011 mega-quake and tsunami triggered the man-made disaster.

All five of the temporary tanks involved in the leaks were collapsible and held together by rubber seals, meaning they were less durable than those with welded seams.

Tepco spokesman Noriyuki Imaizumi said the tank passed a water-tightness test and other safety requirements after it was reassembled, but that the leak might have started when the seals began deteriorating, leading to contortions in the tank.

Tepco has not pinpointed the source of the leak but is concerned that moving and rebuilding it contributed to the incident, rated Level 3 on the International Nuclear Event Scale.

In all, three of the temporary tanks have had to be dismantled and moved because of sunken foundations, the beleaguered utility said. The tanks were relocated in September 2011.

The water in the other two, which are also at risk of leaking, was put in other tanks Sunday.

Nuclear regulators have raised concerns about the flaws of rubber-seam tanks and are urging Tepco to switch to welded-seam tanks, which take longer to assemble and are more expensive.

Tepco said it believes the water went directly into the ground, but that some might have flowed into the Pacific via a rain gutter.

About one-third of the plant’s nearly 1,000 storage tanks are of the rubber-seal type.

Nuclear regulators who toured the crippled plant after the leak was reported declared Friday that the handling of water storage at the site was “sloppy.”

More than two years since the crisis began, Tepco is stumbling badly on the cleanup while the water threatens to spark another environmental disaster.

Calls are growing for outside experts to step in and take control of the operation.