Japan said Monday it would sell off another chunk of the country's massive postal service in a share sale that could raise up to $12 billion.

Tokyo said it would sell up to 990 million shares in Japan Post, or about 22 percent of its outstanding shares, the finance ministry said in a statement.

The sale could raise 1.3 trillion yen based on Monday's closing price of 1,321 yen. The sale price will be decided sometime between September 25 and 27, it added.

After years of delays, 1.43 trillion yen was raised in an initial public offering in late 2015 that included shares in Japan Post's banking and insurance units. 

The bulk of proceeds from selling shares in the government-owned behemoth were earmarked for reconstruction efforts after Japan's 2011 quake-tsunami disaster.

The earlier sale came amid hopes starting to privatise what is effectively the world's biggest bank by deposits could boost investor sentiment and spur efforts to cut red tape in Japan's highly regulated economy.

The sprawling postal group has a network of some 24,000 bureaux across the nation and sits on assets worth more than 290 trillion yen.

The branches also offer services for cash deposits and insurance, and a local branch where many of Japan's ageing retirees withdraw their pension payments.

That system has long drawn criticism both inside and outside Japan, with financial institutions, carrier services and foreign governments arguing that the public body was operating in sectors where it competed directly with private businesses.

The government of former Prime Minister Junichiro Koizumi split the state-owned giant into units in 2007, to handle deliveries, savings, insurance and counter services at each of its post offices.

But the government retained full ownership of the group at first.