The researchers – from the university’s international marine litter research unit – took biodegradable and compostable bags and exposed them to air, soil and sea, all environments they could encounter if discarded as litter.

Biodegradable, oxo-biodegradable and conventional plastic remained functional bags despite being in the soil or marine environment for three years. A compostable bag disappeared from the marine environment after three months, but was present in soil after 27 months. The bag had showed some signs of deterioration.

Research fellow Imogen Napper, who led the study as part of her PhD said: “After three years, I was really amazed that any of the bags could still hold a load of shopping. For a biodegradable bag to be able to do that was surprising. When you see something labelled in that way, I think you automatically assume it will degrade more quickly than conventional bags. But after three years at least, our research shows that might not be the case.”