This Thursday's climate debate is a monumental moment in an election campaign.
The Prime Minister could be "empty-chaired" during a televised climate change debate on Thursday.
Boris Johnson and Brexit Party leader Nigel Farage have yet to respond to a request to attend the hour-long Emergency On Planet Earth debate on Channel 4 News, which will focus solely on climate change.
The broadcaster said the debate will take place even if they are unable to take up the invitation.
Channel 4 has invited seven party leaders to the discussion, but so far only Labour leader Jeremy Corbyn, Scottish First Minister and SNP leader Nicola Sturgeon, Liberal Democrat leader Jo Swinson and Green co-leader Sian Berry have accepted the invitation.
Rebecca Newsom, head of politics at Greenpeace UK, said if the Prime Minister fails to attend the debate it would show the Tories "aren't taking the climate crisis seriously enough".
She added: "This Thursday's climate debate is a monumental moment in an election campaign that should have the climate and nature emergencies at its heart.
"The pitiful 45 seconds given to the topic in the last head-to-head was an absolute joke. And while Boris Johnson acknowledged that the climate emergency is a colossal issue for the entire world, his failure to commit to this 'oven-ready' climate debate raises question marks over his sincerity.
"An empty chair on Thursday night would confirm his party aren't taking the climate crisis seriously enough.
"The Prime Minister cannot afford to run scared of public scrutiny on the defining issue of our generation. This is a test of leadership anyone wishing to run the country must pass."
Ben de Pear, editor of Channel 4 News, said: "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."
Presenter Krishnan Guru-Murthy added: "It is a huge privilege to be hosting the people who want to run the country debating the most important issue in the world.
"How much do we need to change the way we live? Is it the end of fast fashion, fast cars, foreign holidays and red meat? Do any of them have a credible plan to cut our net emissions to zero?"
The debate will take place on Thursday at 7pm and will begin with a short opening statement from each leader.
George Ryan is the PA political reporter.