Liberal Democrat leader Sir Ed Davey is calling on the Conservative leadership candidates to scrap plans for a new coal mine in west Cumbria.
He has written to all five Tory MPs hoping to replace Boris Johnson as prime minister to reject what he calls 'a deeply damaging project.'
A government decision on whether the mine will get the go-ahead is due by 17 August.
Mr Davey said: “The country is burning up but Conservatives are determined to give the go-ahead to new oil drilling and coal mines.
“These projects will damage our climate and treasured countryside even more. If the Conservative leadership candidates cared about protecting our environment they would commit to scrapping these new coal and oil plans now.”
Supporters of the scheme to extract coal from a site near Whitehaven say the mine would create hundreds of new jobs and reduce Britain's need to import coking coal from foreign suppliers. But opponents argue it would have a devastating impact on the environment and climate change.