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Recycling plastic materials "doesn't work" and "is not the answer" to threats to global oceans and marine wildlife, Boris Johnson has said.
Answering children's questions ahead of the COP26 climate change summit, the PM said reusing plastics "doesn't begin to address the problem".
Instead, he said, "we've all got to cut down our use of plastic".
The Recycling Association said the PM had "completely lost the plastic plot".
The association's Simon Ellin told BBC Radio 4's World at One programme Mr Johnson's comments were "very disappointing" and seemed to conflict with government policy.
During the special event organised by Downing Street, Mr Johnson also jokingly suggested feeding human beings to animals to "bring nature back".
He told an audience of eight to 12-year-olds that rather than relying on recycling, people should reduce their consumption of plastic products.
Tanya Steele, chief executive of the World Wide Fund for Nature, told the event: "We have to reduce, we have to reuse – I do think we need to do a little bit of recycling, PM, and have some system to do so."
But the PM said it was a "mistake" to think society can recycle its way out of the problem, and added: "It doesn't work."
Asked later about Mr Johnson's comments, his official spokesman said the PM continued to encourage recycling – though he said relying on it alone would be a "red herring".
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There are plans to increase recycling in England, which the government has said "typically results in lower carbon emissions in comparison to manufacturing products from virgin materials".
"Priority goes to preventing the creation of waste in the first place, followed by preparing waste for reuse; to recycling, and then recovery," the waste management plan for England said.
The most recent figures for England showed a recycling rate of 45.5% for household waste. Waste policy is largely set by the devolved administrations in the UK.
Each UK council collects its plastic recycling differently. A BBC analysis in 2018 showed there were 39 different sets of rules for what can be put in plastic recycling collections.
So what does make a difference?
How many tonnes of carbon dioxide per year each action would save for an average person, according to research by the University of Leeds:
- 2.0 – Live without a car
- 2.0 – Buy an electric car
- 1.9 – Cut out long-haul flights
- 1.6 – Switch to renewable energy
- 0.9 – Go vegan
- 0.6 – Use public transport often
- 0.1 – Recycle
Source: Environmental Research Letters
Ms Steele said she believed "we need to bring nature back" and added: "97% of the mass of mammals on this planet is humans and our animals, our domestic animals. Just 3% is left for the wild."
Mr Johnson said it was "so sad", and quipped: "We could feed some of the human beings to the animals."
Ms Steele replied: "We could have a vote later and ask if there's any candidates."
The prime minister criticised drinks giant Coca-Cola for being among 12 big corporations "that are producing the overwhelming bulk of the world's plastics".
The company sells more than 100 billion throwaway plastic bottles each year, causing problems in countries too small to effectively recycle them, BBC Panorama has found.
Mr Johnson also signalled the government does not want to support new coal mines, as ministers face pressure to prevent a site opening in Cumbria.
And with just days to go before COP26 begins in Glasgow next week, the PM said it was "touch and go" whether the summit would deliver progress.
Millions of lives could be affected by any changes agreed at the conference – and we have devised a quiz for you to discover which policies would have the most impact.
Viewers in the UK can watch the full story on Panorama: Coca-Cola's 100 Billion Bottle Problem on BBC One at 19:35 BST. It will also be available on BBC iPlayer.