Before the tax, Hong Kong people were using an average of about 1,200 plastic bags each a year, or about three plastic bags a day per person.

About 30 million plastic bags a day are thrown away in the city, accounting for about 6 percent of the 17,500 tons of rubbish sent to landfill sites every day.

Previous voluntary attempts to introduce charges on plastic bags in supermarkets had flopped with one chain dropping a pilot scheme because of fierce opposition from customers.

Unveiling the results of its study Thursday, Greeners Action said the tax might have to be increased when the economy rebounds if it is to remain effective.