Biodegradable and compostable plastic bags can still carry full loads of shopping after being exposed in the natural environment for three years.
The finding has come from the University of Plymouth’s International Marine Litter Research Unit, which examined the degradation of five plastic bag materials widely used by retailers.
They said the results cast doubt on whether bags promoted to the public as being environmentally friendly were in fact beneficial.
Bags were left exposed to air, soil and sea, environments which they could potentially encounter if discarded as litter.
Regular monitoring took place for deterioration in terms of visible loss in surface area, disintegration and changes in tensile strength, surface texture and chemical structure.
Results showed that bags described as biodegradable, oxo-biodegradable and conventional plastic remained functional as carrier bags after being in the soil or the marine environment for more than three years.
A compostable bag completely disappeared from the experimental test rig in the marine environment within three months but, while showing some signs of deterioration, was still present in soil after 27 months.
Bags left in the open air completely disintegrated into fragments within nine months.
Research fellow Imogen Napper said the results raised questions as to whether biodegradable materials degraded fast enough to offer any realistic solution to the problem of plastic litter.
She 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.”
Unit head Professor Richard Thompson said the results showed bags labelled as ‘biodegradable’ did not present any ”consistent, reliable and relevant advantage in the context of marine litter”.
He added: “It concerns me that these novel materials also present challenges in recycling. Our study emphasises the need for standards relating to degradable materials, clearly outlining the appropriate disposal pathway and rates of degradation that can be expected.”
A Defra spokesperson said: “The Government is committed to making sure consumers have all the information they need to make informed decisions and be more environmentally conscious when choosing which products to buy.
“That is why we are working to develop a biodegradable standard to give consumers and the recycling industry confidence that things labelled as biodegradable really are.”
Simon Ellin, chief executive of the Recycling Association, said the findings reaffirmed his concerns about compostable, biodegradable and oxo degradable materials and more robust scientific research would be needed “before we jump headlong into a solution that may not actually be a solution, in fact, we may well end up accelerating the process of micro plastics”.
He added: “If we move down this ‘bio’ route for plastics, it detracts from the real problem which is littering and careless disposal of bags and other products. There is a real danger that by advertising the degradation potential of an item, we are moving away from circularity and into a throwaway culture.”
Ellin said the ethos of the resources and waste strategy was that closed loop recycling creates a culture of anti-littering and circularity into which ‘bio ’options did not easily fit.