The Brexit Party leader has already said he will not attend, while Mr Johnson is understood to have asked to send a substitute. This was rejected by Channel 4, which is hosting the event.
Labour leader Jeremy Corbyn, SNP leader Nicola Sturgeon, Liberal Democrat leader Jo Swinson and Green Party co-leader Sian Berry have all confirmed that they will take part.
Channel 4 confirmed to The Independent that it will empty-chair any leaders who do not show up. It refused to comment on reports that it had commissioned an ice sculpture of Mr Johnson to melt during the show if he declines to attend.
The Conservatives are understood to have proposed sending Michael Gove, the Cabinet Office minister, to join the leaders debate in place of Mr Johnson.
However, Channel 4 is insisting that only party leaders will be allowed to take part. A spokesperson said: “Channel 4 News is hosting the world’s first leader’s debate. Michael Gove is not the party leader. The invitation to Boris Johnson remains open.”
Speaking to The Independent, they added: “The clock is ticking. The ice caps are melting. Our invitation to the prime minister remains open until 7pm tonight.”
In an awkward intervention for Mr Johnson, the prime minister’s father, Stanley Johnson, welcomed the climate change debate and thanked Channel 4 for hosting it.
He said: “It is tremendously important that the whole climate and environment issue be fully aired during the general election. There is no doubt in my mind that this is a crucial moment for the environment, both nationally and internationally.
“In just the last two days, the UN environment programme (UNEP) and the World Meteorological Organization (WMO) have highlighted the really disappointing progress made by countries in cutting emissions and what more needs to be done to deal with climate change.”
He added: “I would like to see the next U.K. government place a major focus on climate, the environment and nature protection, and make it part of the central business of the state. So bravo and thank you to Channel 4 for hosting the climate, environment and nature debate.”
Polling suggests that voters also think Mr Johnson should take part.
According to a YouGov survey, more than two in three voters (64 per cent) said he should attend, while just 14 thought he should not.
Even Conservative voters believe he should take part, by a margin of 49 per cent to 27 per cent.
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