Biodegradable and compostable bags have been found intact enough to carry shopping three years after being exposed to air, soil and sea – the same conditions they would encounter if left as litter, a new study has found.

Researchers at the University of Plymouth examined biodegradable and compostable plastic bag materials widely available from high street shops and found none of the bags completely broke down in the environment.

When exposed to the air they all disintegrated into fragments but did not completely disappear. Biodegradable plastic materials remained functional as carrier bags after being in soil or the sea for more than three years, the researchers said.

The compostable bag was found to break down in sea conditions within three months – however it was found still present in soil after 27 months, they said.

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 the most 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.”

Professor Richard Thompson, head of the International Marine Litter Research Unit, added: “This research raises a number of questions about what the public might expect when they see something labelled as biodegradable. We demonstrate here that the materials tested did not present any consistent, reliable and relevant advantage in the context of marine litter.”