Ed Miliband has escalated Labour's attack on the boss of Boots Stefano Pessina, who claimed the Labour leader would be a "catastrophe" as prime minister.
Miliband called on Pessina to start "paying his taxes" and alleged that the chief executive was in an "unholy alliance" with the Tories.
"Now, I've just got to say to you, I don't think people in Britain are going to take kindly to being lectured by somebody who's avoiding his taxes on how they should be voting at the UK general election," the Labour leader told Sky News.
He added: "You've now got this unholy alliance between the Conservative Party and people like him who are actually saying that the country can't change. I think we can change the country.
"I think part of my job is to say: 'We're going to stand up to these powerful forces who aren't paying their taxes, who are avoiding their taxes.' Because frankly, they have a responsibility."
The attack came after Pessina, who is estimated to be worth $11.4bn (£7.5bn) and is the head of Boots' parent company Walgreen Boots Alliance, criticised Miliband.
"One thing is to threaten and to shout, but it is completely different to be in charge and to manage the country day to day," Pessina told The Sunday Telegraph.
"If they acted as they speak, it would be a catastrophe."
The exchange comes with less than 100 days to go before the general election in May.
The latest YouGov poll for The Sunday Times put Labour on a three-point lead over the Tories (35% vs 32%), with Ukip on 15%, the Liberal Democrats on 7% and the Greens on 6%.