A 10p TAX on plastic bags would bring only limited environmental benefits to Scotland, researchers said yesterday.
Full environment gains would be achieved only if paper bags were covered by the levy as well, according to the research commissioned by the Scottish Executive.
It also concluded that a levy on plastic bags was unlikely to make a major dent in Scotland's litter mountain.
"The fact that plastic bags account for less than 1 per cent of land litter suggests that this would have a minor impact on the overall litter problem in Scotland," said the report.
"The same argument also holds for any reduction in the amount of plastic carrier-bag waste going to landfill."
The research was ordered by the Scottish Executive after Mike Pringle, a Liberal Democrat MSP, launched a bill to impose a levy on plastic bags.
His move is being resisted fiercely by the Scottish Retail Consortium and the Carrier Bag Consortium, who say the levy would be an administrative nightmare and would actually increase waste.
The research findings said that although there would be environmental gains, paper bags have a greater effect on the environment than plastic carriers.
"If paper bags are excluded from the levy, as currently proposed, we estimate that paper-bag usage will increase by 174 million bags per year to 213 million per year," said the report.
"This will have associated environmental implications in terms of increased energy use, transport costs, storage space and waste disposal."
The levy would reduce the number of plastic bags used each year by 697 million.
But there would be increased demand for long-lasting bags, bin-liners and paper bags, resulting in an estimated fall of 3,484 tonnes of polythene used in Scotland - but a rise of 8,893 tonnes of paper.
The Scottish Executive has not yet decided its stance on Mr Pringle's bill.