A Rocha has called the dramatic fall a "huge result" for the environment and for good causes, which have benefited from some of the proceeds raised from the tax.
More than seven billion plastic bags were handed out by seven main retailers in 2014, but this figure dropped to just over half a billion in the first six months of this year after the 5p charge was introduced in October last year, the Department for Environment, Food and Rural Affairs (Defra) said.
The charge has also resulted in savings of £60 million in litter clean-up costs and donations of more than £29 million from retailers towards good causes including charities and community groups.
Retailers with 250 or more full-time equivalent employees have to charge a minimum of 5p for the bags they provide for shopping in stores and for deliveries, but smaller shops and paper bags are not included.
Environmentalists have welcomed the move but have called for a more comprehensive scheme which includes all retailers and all types of bags.
Speaking to Premier Andy Lester, from A Rocha, said: "This was a very good example where it was protecting the environment by stealth.
"It was a very small fee for a very large result.
"I think what's taken us by surprise is the amount of revenue that's been generated for really good causes from it.
"It's been really good to see the main supermarkets see this as a victory for them. What a wonderful way of selling your brand by saying: 'we've raised X amount of money for this wildlife charity."
Listen to Premier's Aaron James speaking to Andy Lester:
