Nearly three-quarters of Japanese firms want safety guaranteed before idled nuclear reactors in the country are restarted, seeing no need to rush the process despite the impact the loss of the power source could have on their businesses.

Japan is set to have no nuclear power within weeks for the first time in over 40 years following last year's crisis at the Fukushima Daiichi nuclear plant, which crushed public trust in nuclear power and prevented the restart of reactors shut for regular maintenance checks.

The poll, taken alongside the monthly Reuters Tankan company sentiment survey, showed only 15 percent of firms want an early restart for idled reactors, while 72 percent said safety should be the key priority.

Sixty-five percent of firms think the loss of all nuclear power would hurt their businesses, however, according to the survey of 400 big companies, of which 234 responded from March 30-April 16.

Nuclear power accounted for about 30 percent of Japan's electricity demand before the Fukushima crisis, and the government is eager to avoid a power crunch during the summer when demand peaks.

Trade Minister Yukio Edano this week signalled two reactors idled after the Fukushima disaster would not be back online before the last one currently operating is shut down.

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In a further sign of trouble for Prime Minister Yoshihiko Noda, 58 percent of respondents said conditions were not yet ripe for a hike in sales taxes.

Noda has been struggling to win support for a plan to double the 5-percent tax by late 2015 to fund bulging welfare costs in a fast-ageing society.

Of those wary about tax hikes, 78 percent called for sweeping administrative and political reforms, while 66 percent urged policymakers to present a clear outline for the future of social security. Respondents were allowed to pick more than one area of concern.

The poll underlined public distrust of policymakers who have failed for years to cut wasteful spending and curb public debt now twice the size of Japan's $5 trillion economy - the worst among industrial countries.

The debate on raising the sales tax, one of the lowest among major economies, has long been politically touchy. The last consumption tax increase, in 1997, was blamed by some for triggering a recession that led to a long period of deflation.

The Reuters Tankan showed on Thursday manufacturers' mood worsened slightly in April after a sharp rebound the previous month, reflecting rekindled worries about Europe's debt crisis and concerns the yen could rally again.

(Reporting by Izumi Nakagawa and Tetsushi Kajimoto; Editing by Joseph Radford)