Italian critics say polyethylene bags use too much oil to produce, take too long to break down, clog drains and easily spread to become eye sores and environmental hazards.
Italians use about 20 billion bags a year -- more than 330 per person -- or about one-fifth of the total used in Europe, according to Italian environmentalist lobby Legambiente.
Starting on Saturday, retailers are banned from providing shoppers polyethylene bags. They can use bags made of such material as biodegradable plastic, cloth or paper.
Other European countries have tried voluntary schemes to cut plastic bag use, such as promoting reusable cotton bags. In 2002 Ireland imposed a levy on bags of 15 euro cents (20 U.S. cents) that cut use by 90 percent within a week.
"You are talking of a revolution that is already under way," Legambiente scientific chief Stefano Ciafani said of the shift to biodegradable bags.
Two hundred municipalities out of Italy's 8,000 have introduced their own plastic bag bans, including the cities of Turin and Venice, Ciafani said.
Many supermarket chains have started using biodegradable bags for shoppers even if not on a nationwide basis, Legambiente says on its website.
LACK OF DETAILS
Legislation on the bag ban was set in December 2006 with an original deadline of January 2010. The halt was delayed because of industry opposition but was pushed through by Environment Minister Stefania Prestigiacomo in a blanket decree last week.
Federdistribuzione, Italy's retailers association, said the January 1 deadline could lead to "chaos" and poor service for shoppers given lack of detail in the decree, business newspaper Il Sole 24 Ore said this week.
Italy's rubber and plastics federation estimated the cost of changing over machines to make biodegradable bags was 30,000 euros ($39,440) to 50,000 euros per plant, the paper said.
Shops and shoppers seem prepared. The mid-size Billa supermarket on Milan's bustling Via Torino is ready with white biodegradable bags costing 10 euro cents, twice the price for existing yellow plastic bags, Billa manager Aldo Vismara said.
"We will have them at the check-out from January 1, 2011, and we will replace the yellow ones," he said.
On the downside of the shift, Vismara said there was the possibility the white bag could disintegrate in the rain.
Shoppers worried about the strength of the new bags.
"It's a positive move if the bags are strong enough. The worry is, what happens if they break?" shopper Rosanna said, declining to give her family name.