Compostable bags are to replace single-use plastic bags in more than half of Co-op stores, the supermarket has said.
It says around 60 million plastic carriers will be replaced with the new bags, which will cost 5p and double as a compostable bag for food waste.
Material is certified as compostable if it means it will break down within 12 weeks under specified conditions.
The retailer said the move was a first step towards its plan to "phase out any packaging which cannot be reused".
The compostable bags have been trialled in 22 stores in Greater Manchester since September and will now be introduced in more than 1,400 of Co-op's 2,500 stores.
The bags will initially be in Co-op stores in towns, cities and villages where the bags can be used in food waste collections.
Co-op said it will be speaking with the local councils in the areas where bags cannot currently be used with a view to introducing the bags to more stores.
Shops will continue to sell reusable bags for 10p as well as larger £1 bags, it added.
Its retail chief executive, Jo Whitfield, said: "The first step to remove single-use plastic will be to launch compostable carrier bags in our stores.
"They are a simple but ingenious way to provide an environmentally friendly alternative to plastic shopping bags."
Co-op also pledged to make all its own-brand packaging become easy to recycle by 2023 and to use a minimum of 50% recycled plastic in bottles, pots, trays and punnets by 2021.
What are other supermarkets doing?
Claire Shrewsbury from the sustainability campaign group Wrap said all major retailers had committed to making 100% of plastic packaging recyclable, reusable or compostable by 2025.
"All of them are working incredibly hard and fast to meet those objectives," she said.
Iceland still sells single-use carrier bags in its stores but earlier this year announced plans to remove all plastic packaging from its own-brand products by 2023.