Theresa May will push ahead with plans to double the plastic bag charge, she has confirmed.
A charge of 5p is already made for every new carrier bag in supermarkets.
Now the cost is set to rise to 10p, despite reports of opposition from Chancellor Philip Hammond.
Launching a consultation, the Prime Minister said she would also consider rolling out the 5p bag tax to small retailers, which still supply three billion bags each year.
She said: “We have taken huge strides to improve the environment, and the charge on plastic bags in supermarkets and big retailers has demonstrated the difference we can achieve by making small changes to our everyday habits.
“I want to leave a greener, healthier environment for future generations, but with plastic in the sea still set to treble we know we need to do more to better protect our oceans and eliminate this harmful waste.”
Speaking from Kenya, Mrs May said the 5p charge had led to 13 billion plastic bags being taken out of circulation in the last two years and seen sales in supermarkets drop by 86%.
But Treasury sources are reported to have warned that increasing the charge to 10p could backfire, as it may look as though the Government is trying to “profit” from the tax.
Mr Hammond is reported to prefer tax incentives to charges, not only on plastic bags but also the planned “latte levy” on disposable coffee cups.
Regardless, Mrs May said she was delighted the UK was leading the way on tackling plastics on the international stage.
The Prime Minister has launched a new badge for the Girl Guides and Scouts, intended to inspire young people to become leaders in the fight against single-use plastics.
UN environment head Erik Solheim welcomed the new plastic pollution badge.
“The environment has already paid a heavy price for our addiction to single-use plastics,” he said.
“We simply can’t allow that cost to extend to the next generation.
“This global partnership allows us to not just fight plastic pollution on the beaches, but to invest in the young minds that will preserve the planet for future generations to come.”