Some of Britain's biggest charities have admitted fund-raising practices have "failed to live up" to high standards amid concerns about the tactics used on vulnerable donors.
The heads of 17 charities - including the RSPCA, the Royal British Legion and Oxfam - have signed an open letter in which they insisted "no one should ever feel pressurised into giving".
The Institute of Fundraising (IoF), which co-ordinated the letter, apologised for any wrongdoing by charities and said fund-raisers would be "shocked" if they had caused donors "anxiety or stress".
The charities have spoken out following concerns about the tactics used by some fund-raisers, including the case of 87-year-old widower Samuel Rae who was conned out of thousands of pounds as a result of charities buying and selling his personal details.
In the letter, published by the Sunday Times, the charities said the generosity of the British public placed a "big responsibility on all UK charities to behave well".
They wrote: "We know that there have been times where fund-raising practice has failed to live up to these high standards. We are determined to change that.
"No one should ever feel pressured into giving. The vulnerable should always receive the strongest protection. And we need to act quickly and decisively when any fund-raising practice is found wanting."
They called for a new regulator to impose penalties for charities breaking rules on fund-raising and promised to commit to a strengthened code of practice.
Among the letter's signatories are chief executives of the British Red Cross, Marie Curie, the NSPCC, Cancer Research, Scope, Macmillan Cancer Support, Save the Children and the Alzheimer's Society.
The IoF said it wanted charities to introduce an "opt in" system, preventing them from passing on details of their donors to other charities without the individual's express permission. Information on data selling which is buried in small print should also be banned, it added.
Richard Taylor, chairman of the IoF, said: "I don't know any fund-raisers who wouldn't be shocked if they thought they've created anxiety and distress to members of the public.
"Where that's happened I want to apologise for that and say sorry, we have fallen below the expectations of the public.
"The vast majority of fundraising is done to high standards but I was very surprised and shocked at some of the examples of individual charities and agencies we have seen."
Suzanne McCarthy, the immigration services commissioner, has been appointed as the first independent chair of the IoF's standards committee, the body responsible for setting the rules in the code of fund-raising practice.
A review into the way charities carry out their fund-raising is due to be published on September 16.
The report was ordered by the Government following the suicide of Britain's longest-serving poppy seller Olive Cooke, who received up to 267 letters in a month and regular phone calls from charities asking her for donations.
Responding to the open letter, Sir Stephen Bubb, chief executive of the Association of Chief Executives of Voluntary Organisations (ACEVO), warned that tighter restrictions on fund-raising could cost charities "tens of millions of pounds" in donations.
He told the Press Association: "It's absolutely right that charities are taking action in response to some of the recent publicity around fund-raising techniques.
"But inevitably there are some concerns that less asking means less giving. In the short term, this will undoubtedly lead to a fall in donations and mean great causes those charities support - whether it's research into cancer or supporting refugees - are going to suffer.
"I can't quantify the exact amount of fall but I'm thinking tens of millions, given lots of the main national charities rely heavily on public fund-raising."