The industry ministry is considering limiting each utility's coal-fired power output to around 50 percent of all fossil-fuel energy it generates, sources said Monday.

The rule, designed to curb carbon emissions, would take effect in April as part of the government's efforts to achieve a 26 percent cut in greenhouse gas emissions by 2030 compared with 2013 levels, the sources said.

Under the government's 2030 energy mix goal, coal-generated power is to account for 26 percent of all electricity and liquefied natural gas 27 percent.

But some coal-burning utilities are currently far above the proposed 50 percent threshold, and putting a limit on coal-fired power plants is likely to spur protests from the industry, which has become dependent on them to offset the loss of nuclear power triggered by the March 2011 Fukushima catastrophe.

In the meantime, the retail electricity market is set for full liberalization next April. The restriction proposed by the ministry is intended to prevent new entrants from developing a strong reliance on coal-fired power plants, which can be built at low cost.

But the government may also use it to facilitate the closure of outdated and less-efficient power plants, the sources said.

The government's carbon-reduction goal could be jeopardized unless the power industry, which generates roughly 40 percent of Japan's greenhouse gas emissions, accelerates efforts to reduce output.