The carrier bags, which are designed to break down within three years, leave behind tiny plastic particles that could harm birds, insects and mammals.
The report also warned the bags can litter the countryside for up to five years before they degrade, far longer than the supermarket claims.
This is because they need to be exposed to heat and sunlight for them to break down. And unlike ordinary bags, they cannot be composted or recycled.
The report will be a major blow to Tesco, which hands out around 500million of the '100 per cent degradable' bags a year.