Who says there's nothing but bad news?

Tokyo Electric Power Company workers have detected high levels of radiation in a ditch that flows into the ocean from a leaking tank at the crippled Fukushima Daiichi nuclear plant.

Japan's nuclear watchdog said Thursday the leak could be the beginning of a new disaster — a series of leaks of contaminated water from hundreds of steel tanks holdng massive amounts of radioactive water coming from three melted reactors, as well as underground water running into reactor and turbine basements.

Tokyo Electric Power Co. says about 300,000 litres of contaminated water leaked from one of the tanks, possibly through a seam. The leak is the fifth, and worst, since last year involving tanks of the same design at the wrecked Fukushima Dai-ichi plant, raising concerns that contaminated water could begin leaking from storage tanks one after another.

"That's what we fear the most. We must remain alert. We should assume that what has happened once could happen again, and prepare for more," Nuclear Regulation Authority chairman Shunichi Tanaka told a news conference. "We are in a situation where there is no time to waste."

During the meeting, officials also revealed that plant workers apparently have overlooked several signs of leaks, suggesting their twice-daily patrols were largely just a walk. They have not monitored water levels inside tanks, obviously missed a puddle forming at the bottom of the tank earlier, and kept open a valve on an anti-leakage barrier around the tanks.

TEPCO said the leaked water is believed to have mostly seeped into the ground after escaping from the barrier around the tank. It initially said the leak did not pose an immediate threat to the sea because of its distance — about 500 metres from the coastline. But TEPCO reversed that view late Wednesday and acknowledged a possible leak to the sea after detecting high radioactivity inside a gutter extending to the ocean.

The company also said the tank may have been leaking slowly for weeks through a possible flaw in its bottom. That could create extensive soil contamination and a blow to plans to release untainted underground water into the sea as part of efforts to reduce the amount of radioactive water.

Contaminated water that TEPCO has been unable to contain continues to enter the Pacific Ocean at a rate of hundreds of tons per day. Much of that is ground water that has mixed with untreated radioactive water at the plant.

The watchdog also proposed at a weekly meeting Wednesday to raise the rating of the seriousness of the leak to level three, a "serious incident," from level one, "an anomaly," on an International Nuclear and Radiological event scale from zero to seven.