Businessman Dale Vince is to stop funding Just Stop Oil, saying further protests from the activist group would be "counterproductive".
Mr Vince, a major Labour donor, said further action was "pointless" because the government had shown it would drill for oil "come what may".
Further disruption, he added, would help "feed the Tories' culture-war narrative".
Instead, he said, he would divert funding to the anti-Conservative vote.
Writing for the Guardian, he wrote: "In order to 'just stop oil', first we need to just stop the Tories," adding that he wanted to dedicate his "time, energy and funding" towards increasing the youth vote.
"A vote for anyone other than Labour, or no vote at all, is a vote for another Tory government - this time with a mandate to pursue its anti-green crusade".
Mr Vince said he had supported Just Stop Oil since its foundation in February 2022, and given the group £340,000 to help its campaign against oil drilling in the North Sea.
He said he had always defended the group's tactics, which have included blocking major roads, as well as disrupting high-profile events including Premier League football games and Test cricket at Lord's.
But he added that the government - which granted 100 new North Sea oil and gas licences over the summer - had "made clear that no amount of protest will sway it".
Speaking to OceanNewsUK Newsnight, he said the government would "welcome more disruption," adding Just Stop Oil's tactics had been "weaponised by the Conservative party".
He added it would also feed a "culture war" started by the government with its shift on green policies last month.
"They think they've struck electoral gold here in being anti-green," he said.
He said Labour had not influenced his decision to stop his funding for the group, saying: "this is completely my own decision".
Mr Vince's green energy company Ecotricity has donated more than £1.4m to Labour since 2014, comprising donations to the party itself as well as leader Sir Keir Starmer and deputy leader Angela Rayner.
The company also donated £70,000 to the Liberal Democrats in 2015/16, and £30,000 to the Green Party in 2013.
Labour donations row
Although there is no suggestion Just Stop Oil has funded Labour, Mr Vince's status as a backer of the group has sparked calls from Conservatives for Labour to return donations from him, arguing it legitimises their tactics.
Prime Minister Rishi Sunak has also sought to highlight Mr Vince's support, saying it showed "eco-zealots" from Just Stop Oil are "writing Keir Starmer's energy policy".
Labour has rejected suggestions Just Stop Oil influences its policies, and defended receiving donations from Mr Vince, saying he is a "perfectly legitimate person" to accept money from.
Indeed, in recent months Sir Keir has become increasingly critical of the group, calling its activists "wrong" and "arrogant".
'Rubber-stamping'
Labour has pledged to block all new domestic oil and gas developments, but says it will honour any licences in existence at the time of the next election, expected next year.
The impact of Mr Vince ending his support for Just Stop Oil is unclear. According to its website, most of its money comes from the Climate Emergency Fund - a US network set up in 2019 to fund climate activism.
Just Stop Oil says it also receives donations from the public and other organisations concerned about climate change.
In a statement, Just Stop Oil told the OceanNewsUK it was grateful to Mr Vince for his "amazing financial and moral support over the past year".
The group said civil resistance "really works" - and it believed Labour had "no intention of stopping" the oil and gas projects the government was "furiously rubber-stamping".
"We remain convinced that politics is broken and the Labour Party are moral cowards," Just Stop Oil added.
