Sir Stuart Etherington, chief executive of the National Council for Voluntary Organisations, has written to the Charity Commission claiming that its recent criticism of charities could damage the voluntary sector.
In a letter sent today to Helen Stephenson, chief executive of the commission, Etherington accused the regulator of over-emphasising failures, which he warned could entrench misconceptions and erode trust.
Etherington was responding to a column in The Times newspaper last week by Baroness Stowell, chair of the commission, which coincided with the regulator's report on Oxfam GB's handling of the Haiti safeguarding scandal.
Stowell pledged that the commission would "call out" charities "when they fall short", as she said it had done with Oxfam.
She wrote: "Over recent years, we’ve seen charities losing sight of what they stand for in pursuit of organisational advantage.
"We’ve seen charities engage in pressure-tactic fundraising, supposedly justified by the money that raises for the cause.
"We’ve seen charities that should be working together instead competing for scarce resources.
"And we’ve seen charities putting their reputations before their purposes in responding to failings."
Etherington, who will retire in September, said he agreed that charities needed to "demonstrate the highest standards of behaviour in everything they do".
But he said: "These broad generalisations are far from helpful. Indeed, there is a real risk that they will achieve the opposite effect: they entrench public misconceptions and erode the public’s trust.
"I am consistently hearing from my members that they are concerned about the commission’s direction of travel and its public messaging about charities."
He said the NCVO and many other charities had "taken the lead in improving their practices and raising their standards".
He said: "The commission’s reluctance to recognise these many initiatives is disappointing.
"A confident and effective regulator is one that not only ensures compliance, but also supports charities when they are doing the right thing."
An NCVO spokeswoman said Stowell's column was the latest in a series of "persistently negative" speeches and statements about the sector.
A commission spokeswoman said: "We have received the letter and will respond in due course."