Biodegradable plastic bags can still carry shopping three years after being left in a natural environment.
Five plastic bag materials found in UK shops were tested to see what happens to them in environments where they can appear if littered.
They all disintegrated into fragments after exposure to air for nine months.
But after more than three years in soil or sea, three of the materials, including biodegradable bags, were still intact.
Compostable bags were found to be a little friendlier to the environment - at least in the sea.
After three months in a marine setting they had disappeared, but could still be found in soil 27 months later.
Scientists at the University of Plymouth tested the different materials at regular intervals to see how they were breaking down.
They say the research has raised questions about biodegradable products being marketed to shoppers as alternatives to non-recyclable plastic.
"For a biodegradable bags to be able to do that was the most surprising," says Imogen Napper, who led the study.
"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."
Biodegradable v compostable
If something is biodegradable it can be broken down by living organisms like bacteria and fungi.
Think of a piece of fruit left on the grass - give it time and it will appear to have completely disappeared. In actual fact it's just been "digested" by microorganisms.
It happens to natural substances without any human intervention given the right conditions - like temperature and availability of oxygen.
Composting is the same thing, but it's controlled by humans to make the process faster.
Co-op's compostable plastic bags are meant for food waste, and to be classed as compostable they have to break down within 12 weeks under specific conditions.
The scientists at Plymouth have also questioned how effective biodegradable materials are as a long-term solution to the problem of single-use plastics.
"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.
"It concerns me that these novel materials also present challenges in recycling," said Professor Richard Thompson, head of the International Marine Litter Research.
In the study, the scientists quoted a 2013 European Commission report that suggested about 100 billion plastic bags were being issued each year.
Various governments, including the UK, have since introduced measures like fees to reduce the number being used.