"Stronger results in areas with a higher baseline of plastic bag litter means these policies may be especially effective in these areas," she said.

"It's also very important to keep in mind with the treaty, plastic bag policy addresses just one part of the problem.

"More comprehensive solutions are needed to address it entirely, with a focus on the production side, consumption, and waste," she said.

The research cautions that despite the evidence policies are working to reduce the relative percentage of plastic bags on shorelines, the overall percentage of bags is still increasing in places with and without policies.

This is because plastic pollution continues to grow overall, so policies can only make so much of a dent in the trend.

In the UK, where charges on single-use plastic bags started to be adopted in 2011, a survey found there had been an 80% fall in the number of bags washed up on beaches over a decade.