TOKYO (Reuters) - Confidence among big Japanese manufacturers held steady in the three months to March and is expected to worsen slightly ahead, a closely watched central bank survey showed, in a worrying sign for the government's efforts to boost the economy.

The headline index for big manufacturers' sentiment was unchanged from three months earlier at plus 12 in March, the Bank of Japan's quarterly "tankan" survey showed on Wednesday.

That compared with the median estimate of plus 14 in a Reuters poll of economists.

Big firms plan to cut capital expenditures by 1.2 per cent in the fiscal year that starts April 1, the survey showed. Companies tend to be cautious about capital spending plans at the beginning of a new fiscal year and often revise them up as the year progresses.

The tankan's sentiment indexes are derived by subtracting the number of respondents who say conditions are poor from those who say they are good. A positive reading means optimists outnumber pessimists.