Not enough is known about ability of so-called biodegradable bags to break down, or how long it takes, UK research shows.
The Warehouse Group has announced this week it is switching to compostable bags towards the end of the year, and other retailers are moving to biodegradable totes.
The research, commissioned by the UK Department for Food, Environment and Rural Affairs, follows debate over whether tax exemptions for biodegradable materials should be granted.
Its authors argue current international standards and regional testing is insufficient to predict the biodegradability of carrier bags within waste water, rivers and marine environments.
"In addition to these issues, the toxicological and wider environmental impacts of aquatic plastic debris remain insufficiently understood," the report said.
AUT Engineering professor Thomas Neitzert said the research helped to debunk the belief biodegradable bags were safe for the environment.
Mr Neitzert said as plastic biodegraded, it was still having the same environmental impact, including breaking down into microplastics.
"Until it degrades, it is a fully functional plastic bag with all its disadvantages. It causes problems to marine life and in the process of disintegrating, which takes six months, two years, and longer, it creates smaller particles that can then be ingested by marine life...
"A biodegradable bag can actually never be the end solution to what we're after, which is causing no harm to marine life and [no] pollution on earth and in the water," he said.
Waikato University professor Kim Pickering said the bags were at least a step in the right direction, but there were additives that could be going into the product.
"Unless we know what is exactly in them, then that could be causing problems from the toxicity effects," she said.
Professor Pickering said it was possible to make biodegradable plastics which could be broken down without harming the environment, but some consumers might not be aware of which bags they were buying.
While recycling and upcycling plastics could be seen as better options, in some instances biodegradable materials might be appropriate.
"Where there's bodily fluid contamination, like syringes, for example," she said.
However, there needed to be more responsibility from both consumers and manufacturers on the careless use and disposal of products.