Boris Johnson has said he is "very worried" COP26 might not secure the agreements needed to tackle climate change as he admitted it is "touch and go".
The prime minister, answering questions from schoolchildren around the world, said: "It is going to be very, very tough this summit.
"I am very worried because it might go wrong and we might not get the agreements that we need and it is touch and go, it is very, very difficult, but I think it can be done."
Mr Johnson said the UK has cut CO2 and coal emissions "massively" and he believes the COP26 goals can be achieved.
He added that the world needs "as many people as possible" to get to net zero by the middle of the century.
The PM admitted there was a lot of "peer pressure" at the summit with countries following the example of friends and neighbours, but reiterated: "It's very, very far from clear that we will get the progress that we need."
He welcomed Australia's commitment to reaching net zero by 2050 and said it was "actually very difficult" for the country as it is heavily dependent on coal.
"They've done a heroic thing, the Australians, in getting to that commitment," he told the children as he appeared alongside WWF UK's chief executive Tanya Steele.
He added: "I hope that they will be joined by lots more countries in that region for the COP summit."
Ministers are facing pressure to prevent a new coal mine opening in Cumbria and Mr Johnson gave some of his strongest opposition to the project yet as he said: "We don't want to support new coal mines but what we want to do is to continue our progress to a zero-carbon future."
The PM also, despite saying he would not, named and shamed Coca-Cola as being one of 12 corporations "producing the overwhelming bulk of the world's plastics".
"Big famous drinks companies that you may know, but I won't name. I don't know why not, but I won't name them," he said.
"Coca-Cola, for instance, and others, which are responsible for producing huge quantities of plastic, and we've got to move away from that and we've got to find other ways of packaging and selling our stuff."
Mr Johnson said recycling "doesn't work" as he stressed the need to reduce the amount of the material that is used.
Ms Steele said everyone should reduce and reuse and "do a little bit of recycling".
She also called "to bring nature back" as she said only 3% of the planet is left for the wild.
The PM said it was "so sad", and joked: "We could feed some of the human beings to the animals."
COP26 is being held in Glasgow from this Sunday and the UK government is hoping many world leaders will attend and commit to policies to drastically reduce carbon emissions.
German Chancellor Angela Merkel confirmed on Monday she will be attending the event.