Left-wing activist Owen Jones says he has cancelled his Labour membership and is urging voters to back Green or independent candidates instead.
The Guardian columnist has been a vocal critic of Labour leader Sir Keir Starmer.
In an article for the newspaper, he said a new initiative was being launched to raise money for candidates "seeking transformative policies".
The Labour Party has been approached for comment.
Mr Jones said "those who believe in real change from the Tories' bankrupt model should vote for Green or independent candidates".
"A new initiative – We Deserve Better – is raising money to support such candidates, judged on whether they believe in, say, taxing the well-off to invest, or public ownership, or opposing war crimes, even if they differ on this or that," he wrote.
Mr Jones said he had been a Labour member since he was 15 and as an adult had always campaigned for the party's candidates in local, national and European elections.
"And yet, after a uniquely calamitous 14-year stretch of Tory rule, just as Labour looks set to reconquer No 10 by a landslide, I've just emailed the party cancelling my membership," he wrote.
Mr Jones said his decision came after "a gradual, painful process of realising the party won't even do the bare minimum to improve people's lives, or to tackle the crises that have led Britain to catastrophe".
He added that Labour had become "a hostile environment for anyone believing in the very policies Starmer relied upon to secure the leadership", including scrapping university tuition fees and promoting public ownership.
Mr Jones was a strong supporter of Sir Keir's predecessor as Labour leader, Jeremy Corbyn. When Sir Keir took over in 2020 the columnist said he wanted his leadership to succeed, despite not voting for him.
However, more recently he has criticised him for rowing back on pledges made during his leadership campaign and for his stance on Gaza.
In his article Mr Jones attacked Sir Keir for ruling out bringing back the cap on bankers' bonuses or introducing a wealth tax, as well as committing to "Tory fiscal rules that lock the country into dismal austerity policies that have delivered collapsing public services and an unprecedented decline in living standards".
Labour has argued it needs to be responsible with the public finances and has said that if it wins power it would stick to the government's self-imposed rule that public debt must be falling as a share of the economy by the fifth year of an official forecast.
Mr Jones also criticised the Labour leader's previous comments on the conflict in Gaza last year, when he appeared to say Israel had the "right" to cut off water and energy to the territory.
A spokesman for Sir Keir later said he had meant to say only that Israel had a general right to self-defence.
Labour has faced internal divisions over its position on the conflict, with some criticising the party for being too slow to call for an immediate ceasefire.