Only retailers with 250 or more employees have to charge 5p for bags to reduce the admin burden on small and medium-sized businesses.
The size of the company running the shop determines whether individual stores have to charge. With concessions, the owner's number of employees is the benchmarker.
Defra recommends customers bring their own bags. Paper bags, unwrapped food and uncovered seeds, bulbs and flowers are exempt. Biodegradable bags are not exempt.
In 2013, supermarkets gave seven billion plastic bags to customers. Wales, Scotland and Northern Ireland already have 5p charges. Retailers are expected to donate proceeds to good causes of their choice and have to tell Defra who.