John Caudwell, the founder of Phones 4U, said Rishi Sunak was “an absolute dud” as he praised Keir Starmer’s transformation of his party.
The announcement by Caudwell, who gave the Tories £500,000 at the last general election in 2019, is yet another shattering blow for the prime minister.
It came as a new opinion poll said Labour is on course for a 256-seat majority, with the Conservatives reduced to just 115 seats.
Caudwell said: “For many years now I have been rather despairing about the performance of the party that I have supported for the last 51 years: the Tories.
“Only five years ago, I donated half a million to the Conservatives to help avert the disaster that would have been Jeremy Corbyn in Downing Street.
“But I’ve been increasingly critical of Tory failures since then, particularly over Rishi’s mismanagement of the economy during Covid, Boris’ lowering of ethical standards - and, of course, associated with that the accusation that Tory cronies benefited improperly regarding Covid PPE – and then the Liz Truss debacle.
“Over the last two years especially, I have been amazed by how Keir Starmer has transformed the Labour Party and brought it back from that Corbyn brink.”
He said he had been “delighted” at the Labour manifesto’s focus on growing the economy and making the UK “a clean energy superpower”.
“So, I can declare publicly that I will vote for Labour, and I encourage everybody to do the same,” he said.
“We need a very strong Labour government that can take extremely bold decisions and you can rest assured that I will be doing my best to influence them wherever I can, in putting the great back in Britain.”
Starmer said: “I’m delighted that John, someone with such a successful track-record in business, has today thrown his support behind the changed Labour Party that I lead.
“The message is clear: business backs change and economic stability with Labour, and rejects 5 more years of chaos and decline with the Tories.”