The chairman of the Charity Commission has warned charities not to "hound" people for money through aggressive fundraising techniques.
William Shawcross said it is "unforgivable" for charities to relentlessly pursue people for funds.
His comments come after a year of funding scandals which have shaken the sector's reputation.
Poppy seller Olive Cooke, 92, from Fishponds in Bristol, killed herself in May last year after reportedly receiving hundreds of letters from charities asking for money.
An investigation by The Sun newspaper revealed that Age UK made £6 million from energy giant E.ON for recommending a tariff to pensioners which was not the cheapest on the market.
Mr Shawcross, who will give a speech on charity fundraising in Southampton, told BBC Radio 4's Today programme: "Hounding is absolutely out of the question and unforgivable and charities should never indulge in it.
"We can tell charities that they must not do it, and we do, and I am making a speech in Southampton today saying that the hounding of people by charities is absolutely intolerable.
"I met somebody yesterday who said he had texted £20 to an appeal which he had seen on television and subsequently had hundreds of calls to his mobile phone saying 'would you make that £20 into a standing order?' and so on.
"People must not be treated like that."
He said a new charity fundraising regulator was set up in the wake of claims about Mrs Cooke and bullying fundraising tactics among some charities.
He said: "This will have much more teeth and will give the charities the ability to show they can fundraise in their own best interests, and in the best interests of the public.
"If they don't manage to succeed, then I suspect the Government will give us, the Charity Commission, the task. But I would much rather, and think it's much better, for charities to do it themselves."
The Charity Commission is issuing an alert to trustees at all 1,700 charities, warning them to review their commercial relationships.
The regulator says that while it is legitimate for charities to have commercial partnerships to help them raise funds, trustees must protect the reputation of their organisation.
Charities must check for conflicts of interest, ensure partnerships are properly documented and reviewed regularly, and that the commercial benefits to the charity are made clear.
Those who fail to comply will be subject to regulator action, the Commission warned.