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A courageous teenager who has inspired a generation to fight climate change was called "mentally ill" by a guest on a live news channel today.

Greta Thunberg , 16, accused world leaders of "stealing my dreams and my childhood" in a furious speech delivered at a UN Climate Summit last week .

But today, right-wing political commentator Michael Knowles labelled the youngster a "mentally ill Swedish child" during a discussion on Fox News show The Story. He insisted Greta has depression and obsessive compulsive disorder.

And he refused to apologise when challenged on these comment.

Fox News has since apologised for the "disgraceful" remark and say it has no plans to invite the 29-year-old pundit back.

Mr Trump, meanwhile, retweeted a video of Greta's speech and wrote: "She seems like a very happy young girl looking forward to a bright and wonderful future. So nice to see!"

But Greta hit back by adopting the mantra as her Twitter bio, which now reads: "A very happy young girl looking forward to a bright and wonderful future."

(Image: REUTERS)

Debating on the UN Climate Action Summit of more than 60 world leaders, Mr Knowles, told Fox News : "The climate hysteria movement is not about science.

"It if were about science it would be led by scientists, rather than politicians and a mentally ill Swedish child who is being exploited by her parents and by the international left."

Democrat Party activist Chris Hann confronted Mr Knowles in the show.

"Relax skinny boy, I got this. You’re attacking a child, you’re a grown man, have some couth when you’re on television," Mr Hann said.

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(Image: Getty Images)

"Maybe on your podcast you get away and say whatever you want because nobody’s listening. You’re on national television. Be a grown-up when you’re talking about children.

"She’s trying to save the planet because your president doesn’t believe in climate change and kids need to take to the streets to worry about their future."

A spokesman for Fox News said: "The comment made by Michael Knowles who was a guest on 'The Story' tonight was disgraceful.

"We apologise to Greta Thunberg and to our viewers."

(Image: PA)

It came after Greta, who travelled to the US by yacht to avoid flying last week, gave Donald Trump a withering death stare after her speech in New York.

Addressing the crowd, the activist said: "This is all wrong. I shouldn't be up here.

"I should be back in school on the other side of the ocean yet you all come to us young people for hope. How dare you.

"You have stolen my dreams and my childhood with your empty words."