The first ever election leaders’ debate focusing on the climate crisis will be broadcast by Channel 4 next week, with the prime minister the only major party leader set to be absent.

Channel 4 News said it was awaiting confirmation from Boris Johnson as to whether he would take part and could place an empty chair in the place of the PM if he declines to attend.

The campaigners Possible – who have been pushing for a leaders’ head-to-head on the climate emergency – said it was a historic moment which put global heating front and centre of the election campaign.

Q&A How did the Tories pretend to be a factchecking service? Show During the ITV live leaders debate on 19 November, the Conservative party re-branded their press office account on Twitter as 'factcheckUK', to tweet anti-Corbyn points during the programme to its 75,000 followers. On Twitter accounts there is a username - in this case @CCHQpress - and a screen name, which appears more prominently. The Conservatives changed the screen name to 'factcheckUK', and also changed the logo and biography of the account to read 'fact checking Labour from CCHQ'. No explicit mention of the Conservative party name in full was made, so users would have to know that CCHQ is an acronym for 'Conservative campaign headquarters' in order to understand who was providing the fake fact-checking service. Because the @CCHQPress account on Twitter is 'verified', it means when it appears it has a blue check mark next to the name, to show that Twitter has 'verified' that the account is who it says it is. This was retained while the account was tweeting under the false name 'factcheckUK'. Martin Belam

Max Wakefield, the director of Possible, said: “With poll after poll showing huge demand for emergency action from the political parties this has to be our first climate election.

“It’s in the prime minister’s hands to make this happen, show leadership on the defining issue of our time – and prove he’s ready to take on the challenge if elected.”

Earlier this week leaders from all the main opposition parties wrote to Johnson urging him to take part in any televised debate on the environmental emergency before the election.

The Labour leader, Jeremy Corbyn, the Scottish National party leader, Nicola Sturgeon, the Liberal Democrats leader, Jo Swinson, and the co-leaders of the Green party, Jonathan Bartley and Siân Berry, argued that the public had a right to know what political leaders intended to do to avoid “the irreversible impacts of environmental breakdown”. All have agreed to take part in next week’s debate.

In the letter they stated: “The climate and nature emergencies threaten everything we hold dear; the jobs we do, the health service we rely on, the houses we live in and the food that we grow and eat. The public are right to look to us, their politicians, for leadership. The ambition of our response must match the scale of the challenge.”

The Conservatives, whose environmental record was condemned by leading climate scientists and former government advisers this week, told Possible that Johnson would not participate because he did not want environmental issues to be “siloed”.

The Tory party has been approached for comment.

The idea of a televised climate debate has gained widespread support since it was launched by school strikers, students and pensioners’ groups earlier this month.

More than 500 scientists – including Sir David King, a key government adviser on the climate crisis until 2017 – have backed the plan.

Beccy Speight, the RSPB’s chief executive, said: “We must have the opportunity to scrutinise those vying to be our next prime minister on how they will deliver action at the unprecedented scale required.”

In the past two weeks, the campaign has also received support from more than 70 organisations with a total membership of more than 10 million, including the Women’s Institute, the National Trust and the National Education Union. And more than 188,000 people have signed a petition supporting Possible’s campaign.

The climate emergency has been forced up the political agenda in the past year as growing evidence of the crisis – from floods to wildfires, record temperatures to melting ice – has become impossible to ignore.

In the UK, hundreds of thousands of schoolchildren have taken to the streets to demand action and Extinction Rebellion protests have brought large parts of London to a standstill.