Shoppers in England are now being charged 5p for every plastic bag given out at supermarkets and large stores.
It is hoped the levy will cut the 7.6bn bags given to shoppers at major supermarkets every year, and retailers are expected to spend the money raised from the charge on good causes.
Campaigners have welcomed the move, but called for the law to include all retailers and all types of bags.
England is the last part of the UK to start charging for plastic bags.
The number of plastic bags given out by seven major supermarkets in England rose by 200 million in 2014 to exceed 7.6 billion - the equivalent of 140 per person and amounting to 61,000 tonnes in total.
What do people think?
Brenda Chapman said: "I can see the logic in charging for supermarket bags but if I buy a book or clothes I expect a bag to transport it home in."
Raymond in Bedford said: "Very happy about the charge for plastic bags, they should never have been introduced in the first place. What an environmental disaster they have proven to be."
Katharine in Poole, Dorset, said: "In the 1970s, Safeways used to provide paper sacks for free but charge for plastic carriers. Why doesn't the government revert to something like this?"
The government hopes the English scheme will cut use of plastic carrier bags by up to 80% in supermarkets, and by 50% on the High Street. It also expects to save £60m in litter clean-up costs as well as generating £730m for good causes over the next decade.
The price hike affects only retailers with 250 or more employees, and unlike schemes in other parts of the UK, paper bags are exempt from a charge.
Smaller shops are exempt from the rule change but may choose to charge shoppers for bags as well.
Free bags will still be provided for consumers buying uncooked meat, poultry or fish, prescription medicine, certain fresh produce such as flowers or potatoes, and unwrapped ready-to-eat food such as chips.
The exemptions mean the move may not be as successful as schemes introduced elsewhere in the UK, campaigners have argued
But Alice Ellison, of the British Retail Consortium, said the charge sent out a "confusing, complex message" to customers, adding England should have adopted the same policy as the rest of the UK, where all shops charge for all types of disposable bag.
What is being done elsewhere?
- In 2011, Wales started charging 5p per bag and use by customers has dropped 71%
- Scotland and Northern Ireland introduced their charges in 2014 and 2013 respectively and have also seen significant drops in usage
- In 2002, Bangladesh became the first country in the world to ban thinner plastic bags altogether, after they were found to have choked local drainage systems during floods
A poll for the Break the Bag Habit coalition of litter charities found 62% of shoppers in England - six percentage points higher than in 2012 - thought it was "reasonable" to charge 5p for carrier bags.
Morrisons, Sainsbury's and Asda all said customers were reacting positively to the charge, and it was "business as usual" in their stores.
Friends of the Earth said the charge would significantly reduce the number of plastic bags being used by shoppers, but also called for it to apply to smaller shops.
Environment minister Rory Stewart said it could make a huge difference, "meaning we can all enjoy a cleaner, healthier country".
Regular plastic bags are not biodegradable and can remain in landfill for hundreds of years, Professor Tony Ryan, at the University of Sheffield's faculty of science added.