Climate activist Greta Thunberg has criticised world leaders for what she called their “empty words and promises”, describing their green pledges as “blah, blah, blah”.
The Swedish teenager accused politicians of “shamelessly congratulating themselves” on their plans to cut emissions.
She singled out UK Prime Minister Boris Johnson, echoing comments he made in April.
“This is not about some expensive, politically correct dream of bunny hugging, or build back better, blah blah blah, green economy, blah blah blah, net zero by 2050, blah blah blah, climate neutral blah blah blah,” she told a youth climate event in Milan.
“This is all we hear from our so-called leaders: words, words that sound great but so far have led to no action, our hopes and dreams drowned in their empty words and promises.”
Ms Thunberg was speaking before November's UN Cop 26 climate conference, which is to take place in Glasgow, Scotland.
“Of course we need constructive dialogue," she said. "But they’ve now had 30 years of blah, blah, blah and where has that led us? We can still turn this around – it is entirely possible. It will take immediate, drastic annual emission reductions."
“But not if things go on like today. Our leaders’ intentional lack of action is a betrayal towards all present and future generations.”
Cop26 aims to secure more ambitious climate action from the nearly 200 countries that signed the 2015 Paris Agreement and agreed to try to limit human-caused global warming to 1.5°C.
A UN report in August said the situation was dangerously close to spiralling out of control, with the planet set to face further disruptions for decades to come.
“So-called leaders have cherry-picked young people to meetings like this to pretend they are listening to us, but they are not listening,” Ms Thunberg told the youth summit.
“There is no planet B … Change is not only possible but necessary, but not if we go on like we have until today.”
At a pre-Cop26 meeting beginning on Thursday, about 50 climate ministers will address key challenges, including differing views on the pace of transition and who pays for it.
“Now is the time for leaders of the biggest economies and the biggest greenhouse-gas emitters to make much bolder commitments,” Cop26 president Alok Sharma said on Tuesday.
Wealthy countries pledged a decade ago to mobilise $100 billion a year to help vulnerable nations adapt and transition to cleaner energy, but are still short of their 2020 goal.
“It was promised by 2020 and we are still waiting,” Vanessa Nakata, a youth delegate from Uganda, told the Milan event.
“How long must children sleep hungry because their farms have been washed away, because their crops have been dried up because of the extreme weather conditions?”
“How long are we to watch them die of thirst and gasp for air in the floods? World leaders watch this happen and allow this to continue.”