Boris Johnson is refusing to take part in the first ever election leader’s debate focusing on the climate crisis, with Channel 4 threatening to leave an empty chair if the prime minister does not attend.

The broadcaster said Mr Johnson had yet to respond to a request to attend the hour-long Emergency On Planet Earth debate on Thursday.

It came hours after the UN published a report warning that countries would need to increase their carbon-cutting efforts five fold to avoid climate chaos.

Channel 4 has invited six party leaders to its climate debate, but so far only the Labour leader Jeremy Corbyn, the Scottish First Minister National party leader Nicola Sturgeon, the Liberal Democrats leader Jo Swinson, and the co-leader of the Green party Sian Berry, have agreed to take part.

It is understood that the prime minister is not willing to debate with Ms Sturgeon because she is not a candidate at the election, and it is not clear if a replacement will be sent.

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The Brexit Party confirmed its leader, Nigel Farage, would not be taking in the debate as they “have no faith that the broadcaster will conduct the debate in a fair and objective way”.

But Channel 4 said the debate would go ahead even if Mr Johnson and Mr Farage refused to attend.

The climate campaigners Possible, who launched a petition calling for a leaders’ debate on the climate crisis, said the prime minister risked being on the "wrong side of history" by snubbing the debate.

Max Wakefield, the group's director, said: "The world’s first climate and nature TV leaders' debate is a defining moment - a unique platform to end the relentless sidelining of this emergency.

"This is bigger than any party - which is why we need all of them there. The prime minister must choose to join other party leaders or face being on the wrong side of history."

Writing in The Independent, Green MP Caroline Lucas urged Mr Johnson to reverse his decision to snub the debate and “show that his party has more than reusable straws and drinks stirrers as a response to the greatest crisis of all time.”

“Failing to show up for it won’t just make the prime minister a ‘climate chicken’ – it will reveal his utter contempt for the many, many people in the UK and abroad who are facing this desperate climate emergency and looking to our leadership for answers,” she said.

Ben De Pear, editor of Channel 4 News, said the invitation remained open to the PM and Mr Farage,“but this debate will go ahead with or without them."

He added: “There is no more urgent issue facing the planet and we are delighted to open the whole of our programme for all the party leaders to show what plans they have to confront it.”

The Tory party has been approached for comment.

The Conservatives' climate record has been condemned by an investigation that found it had failed on “pretty much every aspect” of protecting wildlife and the environment.

The party's plans for achieving their net-zero pledge to end carbon emissions by 2050 have also been criticised by green groups for lacking urgency and practical solutions.

The climate emergency has been forced up the political agenda in the UK after hundreds of thousands of school children took part in climate strikes and Extinction Rebellion brought parts of London to a standstill.

The Channel 4 debate comes a week before the UN climate talks in Madrid, which seek to advance a global strategy for tackling climate change. A number of reports have been released ahead of the talks warning of record levels of greenhouse gasses in the atmosphere, with fossil fuel production on track to exceed safe climate change limits by 120 per cent.