A senior minister said the government saw no alternative to providing continued support for the utility, known as Tepco.

Tepco officials suggested the costs of compensation and decontamination could double to 10 trillion yen ($124.55 billion), making greater government support vital.

"If the costs ballooned to 10 trillion yen, double our estimate of a few months ago, we could not shoulder such a financial burden," the company said in a statement.

An earthquake and tsunami in March 2011 triggered meltdowns and radiation leaks at the Fukushima Daiichi plant, 150 miles northeast of Tokyo, causing widespread contamination and prompting mass evacuations.

Tepco, which provides power to about 45 million residents of the Tokyo region, has been drawing on a 5 trillion yen fund to pay costs. It also received a 1 trillion yen capital injection in July to avert bankruptcy.

"It is unavoidable that we will have to revise the current financial support framework," Tepco Chairman Kazuhiko Shimokobe told a news conference.

Under Tepco's business turnaround plan announced in May, Japan's largest utility by revenue must eventually pay back funds received from the government.

Minister for National Policy Seiji Maehara, addressing a separate briefing, said: "For the foreseeable future Tepco will remain under the control of the government and we will have to support the company."

DIFFICULT TO ESTIMATE COSTS

Masashi Goto, a retired power plant designer and university lecturer, said technological and other uncertainties made it hard to estimate final costs of clean-up and decommissioning.

"It also appears that Tepco underestimated the cost involved at first, probably because it would be criticized by the public by admitting to the immense scale of the catastrophe," he said.

He said costs were likely to increase as decommissioning work, likely to take decades, progresses.

Tepco has said it will have to resort to major rate hikes again if the government fails to provide additional support. It also said on Wednesday it would set up a coordinating centre in Fukushima, with 4,000 staff, to oversee clean-up operations.

Tepco acknowledged for the first time last month, in a document outlining company reforms, that it had failed to anticipate and tackle the March 2011 disaster.

It said it had feared that implementing accident measures would alarm the public and boost Japan's anti-nuclear movement.

The company is banking on restarting its undamaged Kashiwazaki-Kariwa nuclear plant, the world's largest, to help it return to profit in the business year starting in April 2013.

But prospects appear dim for restarting its seven reactors gradually from April 2013 as safety standards for restarts, to be drawn up by Japan's new nuclear watchdog, will probably not be issued until next year.

All but two of Japan's 50 reactors have been halted for maintenance and safety checks to see if they could withstand an earthquake and tsunami similar to last year's disaster, the worst atomic crisis since Chernobyl in 1986.

($1 = 80.2900 Japanese yen)

(With additional reporting by Mari Saito; Editing by Aaron Sheldrick and Ronald Popeski)

• A Permanent Economic Emergency