The European Commission is considering introducing better ways of labelling biodegradable and compostable bags.
Compostable bags only biodegrade in industrial composting plants. Biodegradable bags will biodegrade in the natural environment, but come in different types:
Those made of corn will biodegrade in a landfill environment, but while doing so they produce methane, a powerful global warming gas
Another type of bag is oxo-biodegradable, which will biodegrade if exposed to air or water, but not in landfill
Symphony, a British company which manufactures oxo-biodegradable bags, claims it can be "programmed" to biodegrade within six to 18 months.
Symphony's chairman, former Conservative MP Michael Stephen, told the BBC: "There is a huge patch of plastic roughly the size of Texas swirling about in the north Pacific. If it had been oxo-biodegradable it would have disappeared by now."
Mr Stephen said: "Ireland and Wales missed an opportunity to make all the remaining plastic oxo-biodegradable."
But Ms Romer, of plasticbaglaws.org, says: "The term biodegradable is often misleading, which prompted a bill recently signed into law in California prohibiting the sale of most plastic products labelled as 'biodegradable' unless the claim includes a disclaimer."
Mr Stephen agrees: "Suppliers need to be clear as to what kind of biodegradable plastic they are offering and what it will, and will not, do. It would for example be misleading to describe most compostible plastics as biodegradable.