Peter Kyle, deputy chief executive of Acevo, has accused trade union Unite of launching a concerted attack on the voluntary sector.
Kyle, whose organisation represents charity chief executives, was angered by recent Unite statements that he believes undermine the sector's role and potential for delivering more public services.
Last week the union said charities were too financially unstable and paid their staff too little to be able to compete for NHS health contracts.
Early in November it said the "excessive City pay culture" had corrupted the voluntary sector's ethos, and in October it criticised plans to turn NHS services into social enterprises.
Kyle said the attacks were "insulting and ignorant" and accused Rachael Maskell, Unite's national officer for the not-for-profit sector, of waging a personal vendetta.
"She fundamentally misunderstands the role and motives of people in the third sector," said Kyle. "People who work for charities want to achieve social change as much as those in any other sector."
He asked why other unions had not spoken out on these issues recently.
"I don't think her views represent the views of her members, and certainly not the union movement as a whole," said Kyle. "This is a concerted attack on the third sector by Unite."
Maskell denied the allegations, saying it was her job to raise members' concerns about issues such as pay inequality and job losses.
"We're a campaigning organisation and it's our duty to speak up for members," she said.
"I'm not sure what Peter Kyle is on about because everything we do is about ensuring good quality services and jobs, and a vibrant third sector."