Richard Walker, executive chairman of the UK's 10th largest food retailer Iceland, who quit the Conservative Party in October, wrote in the Guardian newspaper that Labour and Starmer were best for a population suffering cost-of-living pressures.
Labour has been courting companies before an election later this year, winning support from a growing number of investors, executives and bankers who say the Conservatives have lost their traditional stance as the party of business.
"After a lot of soul-searching, I am now clear in my own mind that Labour is the right choice for the communities across the country where Iceland operates - and the right choice for everyone in business who wants to see this country grow and prosper," said Walker, whose chain often serves poorer regions.
"Having met the man, I am sure that Starmer has exactly what it takes to be a great leader," added Walker, who once tried to run as a Conservative lawmaker.
"He absolutely gets it when I talk to him about the way that the cost of living crisis has put unbearable strain on the finances of so many of my customers and their families."
Labour is urging supporters to help boost its coffers after the Conservatives received from donors almost three times as much as the opposition party in three months to September last year, according to latest funding data.
Starmer, who now describes Labour as the party of business and will visit an Iceland store in northwest England on Monday, was delighted at Walker's decision. "With Labour, shoppers and shop workers will get a fair deal," he said in a statement.
(Reporting by Elizabeth Piper; Editing by Andrew Cawthorne)
