Labor received a £1million donation from Lord Sainsbury's daughter, the party's biggest ever donation from a female supporter.
Fran Perrin said she gave the huge sum to help Keir Starmer and shadow chancellor Rachel Reeves restore “Britain's place in the world” after the damage caused by the Conservatives.
The philanthropist also attacked “13 years of bad decisions and reckless choices” by the Tories – saying the UK deserved better than the “self-obsession” shown by Rishi Sunak's party.
The business intervention represents the latest blow to the Tories since the towering Ken Clarke threw his weight behind Ms Reeves as chancellor following a resounding endorsement from former Bank of England governor Mark Carney.
Mr Sunak is a steady stream of defections from top Tory donors and business defectors to Labour, with polls putting Sir Keir's party on course for a general election victory similar to Sir Tony Blair's.
Ms Perrin, founder and director of the charity Indigo Trust, said: “I have never been more sure that Labor is the best choice for our future.”
She said The Telegraph: “I think Keir and Rachel are the best chance this country has to renew our place in the world. Every cause I care about needs Labor in power if anything is going to improve.”
Ms Perrin, who has previously acted as a government adviser under the Labor governments of Tony Blair and Gordon Brown, said: “There is so much damage that needs to be undone after 13 years of bad decisions and reckless choices.
He added: “Britain deserves better than Tory self-interest, self-obsession and playing with people's lives.”
Her father, Lord Sainsbury, is believed to have given Labor their biggest ever personal donation when he donated £3million earlier this year.
The former supermarket chairman withdrew his financial support for the party under Jeremy Corbyn. The peer said he wanted “to see Keir Starmer become Prime Minister as soon as possible”.
The roughly £2m Ms Perrin has given to Labor in recent years puts her on par with Lubov Chernukhin, the Russian-born UK businessman who has donated just over £2m to the Tories.
Welcoming the latest major donation, a Labor spokesman said: “Donors are coming back to Labor because they see that we are a changed party that is serious about delivering for working people with five missions to transform Britain.
They added: “Labour is a changed party that will work hard to take back Britain's future.”
Earlier this month, Richard Huntington, the chief strategist at the Saatchi & Saatchi agency behind the pre-1979 ‘Labour don't run ads' – said The independent that Britain needs to be saved from “another five years of stagnation, austerity and despair”.
The executive chairman of Icelandic food chain Frozen, Richard Walker, became the latest Tory donor to withdraw his support last month, saying he was “open” to backing Sir Keir.
Phones4U founder John Caudwell has said he will not back the Tories after the “madness” of Mr Sunak's net zero shift – and is considering giving to Labor instead.
Another Tory donor, Philip Harris, founder of Carpetright, said the Conservative Party did not “deserve” to win the 2024 general election.