The multimillionaire said the prime minister and chancellor were practising “GCSE economics” but described Labour as “sensible people who have got their feet on the ground”.
“The Tories’ behaviour over several years has made the UK a laughing stock,” Mr Quarry told The Times.
He added: “What’s dawned on me, particularly since the Johnson years, is that there is a desperately growing inequality which just seems unfair. I think people need hope. They need hope that their personal situation will improve, they need hope that they’ll get a break.”
Angry at the radical, borrowing-fuelled tax cuts, the donor said Ms Truss and Mr Kwarteng “a game of roulette where they’re just betting everything on one number coming up in the next spin”.
Mr Quarry – a former City lawyer who chairs the legal recruitment giant SSQ – said his £100,000 donation to Labour was “very much the start of a process”. He has also become a party member.
The entrepreneur met shadow chancellor Rachel Reeves and other members of Sir Keir’s team at the conference in Liverpool. “I do think Starmer and his colleagues really do want to build a very close partnership between government and business.”
He previously donated around £75,000 to the Tories under the leadership of David Cameron and Theresa May, and reportedly joined exclusive “leaders’ group” donor events with senior ministers.
It comes as Ms Truss and Mr Kwarteng abandon their plans to axe the 45p tax rate for top-earners after a Tory revolt, in a humiliating U-turn at the Tory conference.
“We get it and we have listened,” Mr Kwarteng said, calling the 45p rate cut “a distraction from our overriding mission to tackle the challenges facing the country”.
Watch: 'Have you considered resigning?' - Kwasi Kwarteng grilled over 45p rate tax cut
No 10 would not be drawn on whether Ms Truss or Mr Kwarteng pushed for the U-turn, saying they both “accepted that this was a distraction”.
The prime minister’s official spokesman said she remained was committed to the rest of Budget, and was confident it could be passed through parliament.
The U-turn will reduce the unfunded tax cuts in the chancellor’s mini-Budget by just £2bn, from £45bn to £43bn, said Paul Johnson, the director of the Institute for Fiscal Studies (IFS) – who called the move “trivial”.
The IFS chief told BBC Radio 4’sToday: “To the extent that what we saw a couple of weeks ago was leading to fiscal unsustainability, it still is. Nothing really has changed.”
Mr Johnson added: “The Bank will be just as concerned that £43bn is being pumped into the economy as it would have been with £45bn.”
Former Tory minister Stephen Crabb said the U-turn will not “draw a line” under Tory MPs’ rebellion, saying many will still worried about cuts to benefits and public services.
However, former cabinet minister Michael Gove – having expressed his own anger at the 45p rate abolition – has indicated he is now willing to vote for the mini-Budget.
“I will want to support and I think, on the basis of everything I’ve heard, there’s lots that can be enthusiastically supported,” he told Times Radio.
But Mr Gove added that he would need “a lot of persuading” to support the government if Ms Truss decided not to uprate benefits in line with inflation – a move likely to be voted on separately from the mini-Budget.
“I wouldn’t want to prejudge an argument … sometimes have to do things and embrace policies that would in other circumstances be deeply unattractive,” he said. “But my basic position, my starting position is, yes, Boris was right [to pledge to uprate benefits in line with inflation].”